(CNN) The political turmoil surrounding US health care policy has reached fever pitch in Washington.

US Republicans have been working for seven years to topple Obamacare and on Thursday those efforts were dealt a devastating blow after a failed late-night Senate vote.

But why is Obamacare so controversial and health care such a hot-button issue in the United States?

Here's what you need to know to get caught up.

What is Obamacare and why do Republicans want to overturn it?

The Patient Protection and Affordable Care Act, known as Obamacare or the ACA, became law under former US President Barack Obama in 2010.

It was a massive overhaul of US healthcare that made it illegal for private health insurers to deny coverage because of pre-existing conditions. It also expanded Medicaid coverage to adults on low incomes who couldn't afford to buy health insurance.

It also mandated that all Americans have health insurance -- known as the "individual mandate" -- or pay a penalty -- something the Obama administration believed necessary to make the economics work: young, healthy people needed to pay into the system to offset the costs of the sick and the elderly.

Republicans say the legislation places too many burdens on businesses and hinders job creation. They believed legally requiring Americans to pay for a service they may not want was a gross infringement on individual liberties.

They tried to knock down President Obama's legislation several times but he was able to block their efforts while still in office.

President Trump campaigned on a pledge to repeal and replace "the disaster that is Obamacare" - - and Republicans have been doing their best to do this since he took office -- but with little progress.

What do Republicans want to replace it with?

In the early hours of Thursday, the Senate voted on a slimmed down plan that would have repealed parts of the Act -- known as a "skinny repeal."

It would have eliminated the "individual mandate" and it would have removed the mandate that requires employers provide affordable coverage for eight years.

Why is there so much drama?

Thursday night's dramatic turn of events was the culmination of months of painful negotiations.

It marked a long-awaited opportunity for a legislative victory for President Donald Trump that would have fulfilled a seven-year Republican promise to overhaul the Affordable Care Act.

Hours before the failed vote Trump tweeted: "Go Republican Senators, Go! Get there after waiting for 7 years. Give America great healthcare!"

Go Republican Senators, Go! Get there after waiting for 7 years. Give America great healthcare! — Donald J. Trump (@realDonaldTrump) July 28, 2017

Sen. John McCain, who was recently diagnosed with brain cancer , didn't disclose his vote until the end, and was lobbied by Vice President Mike Pence and others on the floor as it became clear he would oppose the plan. Trump also spoke to him in a final, unsuccessful effort to get him to vote yes

The vote ultimately failed 49-51, with Senators Susan Collins and Lisa Murkowski the other two Republican no votes.

Heightening the sense of drama, lawmakers had been preparing for a potential vote-a-rama -- an exhausting Senate process which allows any senator to introduce any amendment they want, although the no vote removes that prospect from the table.

How does US health care stack up with the rest of the world?

Many outside the US find the health care saga bewildering. In the UK, health care has been free and universal since 1948, when the country's National Health Service was established, funded via compulsory contributions and taxes.

The US system is the most expensive in the world, with 17.1% of GDP spent on healthcare in 2014 -- a far greater slice than other developed economies.

What does it mean for ordinary Americans?

For now, Obamacare remains intact.