Washington (CNN) The Clinton Foundation is under intensifying scrutiny as Republicans accuse it of being a corrupt and opaque institution.

The sprawling, complex structure of the foundation, its history of contributions from foreign power brokers and its attractiveness as a Republican line of attack mean it's likely to stay in the spotlight.

But the glare of public attention hasn't always been so enlightening about the nature of the foundation and its activities. Here's a closer look.

What is the Clinton Foundation?

Former U.S. Secretary of State Hillary Clinton, vice-chair of the Clinton Global Initiative Chelsea Clinton and founder of the Clinton Global Initiative Bill Clinton onstage during the fourth day of the Clinton Global Initiative's 10th Annual Meeting at the Sheraton New York Hotel & Towers on September 24, 2014 in New York City.

Bill Clinton set up the public charity after he wrapped up his presidency in 2001 with the idea of bringing government, businesses and social groups together to tackle big problems. It was kind of a new idea at the time. On Monday, Clinton wrote in a post on Medium that the foundation is about "creating opportunities and solving problems faster, better, at lower cost so that more people are empowered to build better futures for themselves, their families and their communities."

But ... what is it?

The foundation is made up of 11 non-profit groups that work on four major issues: global health and wellness, climate change, economic development and improving opportunities for girls and women.

How does it work?

It's what philanthropists call an operating foundation, which means it doesn't give much money to outside groups to fulfill its mission. In 2014, the foundation gave less than 3% of revenue to other non-profits. Instead, the foundation does its own work, with staff and partners around the world.

What kind of work?

Health is a big focus. In more than 70 countries, according to the foundation, it helps 11.5 million people, including 800,000 children, with HIV/AIDS get their medication at 90% lower cost -- more than half the adults and three-quarters of the children getting treatment in the world today.

But it does all kinds of other work as well. For instance, it helps East African farmers get better seeds and fertilizers. It supported Nepal's reconstruction after the 2015 earthquakes. And it has connected more than 500,000 Latin Americans to job training and entrepreneurship opportunities.

Okay. So it's an international do-good group?

Yes. And no. The Clinton Foundation does tons of work in the US, too. Some examples: It has a school program that operates in every state, affecting more than 31,000 schools and 18 million students by its count. That program is to improve physical education, child nutrition, health education and staff wellness programs. They also work on prescription drug addiction. The foundation wants to halve the number of opioid overdoses -- right now those drugs kill more Americans than car accidents.

Is the foundation the same thing as the Clinton Global Initiative?

The Clinton Global Initiative is part of the foundation. Bill Clinton started CGI in 2005. In contrast to other parts of the organization, it doesn't fund or manage projects, or handle any of the money involved. Instead, it's like the OKCupid of the charity world, matching funders with good causes. They do this at events throughout the year, including a big annual meeting in New York every September that's a who's who of global movers-and-shakers. (Bono? Check.)

So ... lots of charity hook-ups?

Since the first annual meeting in 2005, CGI says it has helped make more than 3,500 pledges, or "commitments," between funders and projects. In real-life terms, that means more than 430 million people in more than 180 countries have had their lives touched.

More than 46 million children have gotten access to a better education, the foundation calculates. CGI says it has raised more than $313 million to research and develop new vaccines, medicines and diagnostics. And it has helped more than 27 million people have better access to safe drinking water and sanitation.

If the Clinton Foundation is doing all this good, why the suspicion?

Because of concerns about transparency and -- especially after Hillary Clinton became secretary of state -- concerns that some donors, especially foreign governments, may have been trying to buy influence.

The AP reported Tuesday that during her time as secretary, more than half of Clinton's meetings with people outside government were with donors to the Clinton Foundation. These included executives at Estee Lauder, which was working with the State Department on a gender violence project in South Africa, and the founder of a non-profit bank who was asking for help because the government of Bangladesh pressured him to resign.

What did the State Department have to say about that?

The deputy spokesman, Mark Toner, said in a statement that "individuals, including those who have donated to political campaigns, non-profits, or foundations -- including the Clinton Foundation -- may contact or have meetings with officials in the administration. A wide range of outside individuals and organizations contact the State Department. Meeting requests, recommendations and proposals come to the department through a variety of channels both formal and informal."

What's the issue with transparency?

Who the foundation was getting money from, basically. When Clinton took the State Department job in 2008, she promised President Barack Obama that the foundation would publish all its donors every year. And ... that didn't happen.

Reuters discovered that from 2010 to 2013, the foundation's health arm wasn't disclosing all of its donors -- leaving out countries like Switzerland and lumping together individuals as one big group. And the foundation didn't tell the State Department that countries such as Australia and the UK doubled and tripled their donations between 2009 and 2012 while Clinton was secretary.

What do Clinton's opponents have to say about all this, you ask?

No surprise, Trump has been hitting this theme hard, trying to use the foundation to undermine Clinton. This week in Akron, Ohio, he reeled off accusations without offering proof, saying that, "No issue better illustrates how corrupt my opponent is than her pay-for-play scandals as secretary of state."

He's calling for a special prosecutor to investigate the foundation.

Is it only Republicans who rap the foundation?

No. There's been a lot of media coverage of possible "pay-for-play" deals while Clinton was at state. So far, though, there are no smoking guns. The Wall Street Journal has said that the foundation has benefitted Clinton friends. And Fox News raised questions about the State Department's consideration of a Nigerian land purchase in March 2013 from two Lebanese-Nigerian brothers who donated heavily to the Clinton Foundation. Clinton left the State Department on February 1, 2013, and the real estate deal never took place.

Any Democratic critics?

In a June 2016 interview with CNN's Jake Tapper during primary season, Vermont Sen. Bernie Sanders, Clinton's only remaining Democratic challenger, said that foundation donors like Saudi Arabia created the appearance of a conflict of interest. "Do I have a problem when a sitting secretary of state ... collects millions of dollars from foreign governments ... dictatorships," Sanders said. "Do I have a problem with that? Yeah, I do."

Is it weird that the Clinton Foundation has foreign donors?

It turns out that foreign donations to US foundations and charities are common.

"Like other nonprofits, the Clinton Foundation is chasing after grant money from the real foundations that have it, like Gates and Rockefeller, as well as foreign governments, most of which also donate to other major nonprofits," Inside Philanthropy's editor in chief David Callahan has written.

What do the Clintons say in response to all these accusations?

The foundation has always rejected the "pay-for-play" accusations. The health arm of the foundation said there were various reasons for not disclosing all its donors. Bill Clinton said The Wall Street Journal story about the foundation benefitting his friends wasn't accurate. And foundation spokesman Craig Minassian has said that Clinton's friends are involved in CGI commitments because they also care about making a positive impact.

So what happens if Clinton becomes the next president?

"The process of determining the Clinton Foundation's future if Hillary becomes President has not been easy," Bill Clinton wrote in his Monday Medium post. "It's an unprecedented situation, so there's no blueprint to follow."

That said, he laid out some steps the foundation will take if his wife wins. It will only accept donations from US citizens, legal residents and US-based independent foundations, he wrote. Bill Clinton will step down from the board and stop fundraising.

Because a lot of the international work the foundation does is partially funded by foreign governments' aid programs, they'll move those programs to other organizations so they can continue. And, Clinton added, this September will mark the last annual meeting of the Clinton Global Initiative.

How does the philanthropic world see the Clinton Foundation?

They're held in high esteem. There are watchdog groups that judge charities on how they're run, how transparent they are and how much they spend on programs -- some charities raise a ton of money, but spend a large percentage on salaries and bonuses instead of their actual cause.

Charity Watch gave the Clinton Foundation an A grade, while GuideStar gave it a platinum rating.

Daniel Borochoff of Charity Watch noted that in 2014, 87.2% of the foundation's funding went to its programs, "which is really high." The foundation, he said, does "really important, valuable work that saves lives of lots of people."

For a while, another group called Charity Navigator had the Clinton Foundation on a watch list because of media reports about possible conflicts of interest. It didn't judge the merit of the reports but wanted to flag for donors that others were raising questions.

How does the charity world see the political brouhaha over the foundation?

"It's unfortunate that it's become this punching bag, this political punching bag," Borochoff said. "There's a lot of things that are said that are false. If Hillary Clinton wasn't running for president, the Clinton Foundation would be seen as one of the great humanitarian charities of our generation."