Think of it as the Pinterest of to-do lists.

Trello is an online tool for managing projects and personal tasks. That may sounds rather prosaic. But this increasingly popular app often inspires the sort of passion usually reserved for consumer apps like Pinterest or Instagram. It's the kind of business software that slips into businesses through the backdoor, just because individual employees like how it works. "Love it as a non-sanctioned work management system," began one of the many responses we received when asking about the tool over Twitter in recent days. "So versatile, fast."

Unlike a typical task list—which tend to just be lines of text with little check-boxes next to them—Trello brings a sense of physicality to project management. Each task is a virtual "card" that can be dragged and dropped between different columns, and that's no small thing.

This visual approach has attracted 4.5 million users since its launch in 2011, despite facing thousands of competitors, including Microsoft Outlook, Facebook co-founder Dustin Moskovitz's web app Asana, and even its parent company's own flagship product.

Now, this parent company, Fog Creek Software, is trying to take this tool to the next level. Last week, it spun the Trello business into a separate company after raising $10.3 million dollars. And despite playing in a crowded market, the tool just might have the sort of unique approach that will allow it spread in a very big way.

A Tool for 'Civilians'

Trello is dead simple, and it's used by people of all stripes. But its origins are deeply geeky. Programmers Joel Spolsky and Michael Pryor founded Fog Creek in 2000. The two had been working for the internet service provider Juno in New York City, but wanted to try their hands at product development. Though there were plenty of financial services companies hiring programmers, Spolsky says, there weren't many software companies in New York City at the time. So they started their own company, based the idea of attracting the best developers in the business and treating them well.

From left, Michael Pryor and Joel Spolsky. Trello

That doesn't sound unusual in today's job market where developers are treated like rock stars, but Spolsky says that at the time developers were rarely treated well in New York. Instead of taking outside capital, the pair funded their product development efforts by doing consulting for other companies, and for the first 10 years of its existence, Fog Creek focused on creating products specifically for other software developers, such as its flagship product FogBugz, a software project management application. But then, in 2011, Pryor and Spolsky decided to try creating a product that would have appeal to non-developers, who they refer to as "civilians."

After ten years of work, Fog Creek had enough cash on hand that it could experiment with new products that didn't bring in revenue immediately. The company assigned eight developers to work on four separate, experimental products. Spolsky describes it has being like having four "mini Y Combinator companies" inside Fog Creek, referring to the famous Silicon Valley startup accelerator. Trello was one of those projects.

The Secret of Its Success

The secret of Trello's success, Spolsky says, is that although it's flexible enough to be used for a whole range of projects—from software development to job applicant tracking to wedding planning—it's also deliberately constrained. "It's not easy to put a lot of things into Trello," says Spolsky. "It's meant to keep you focused on the things that are important right now, rather than way of building this backlog of items that you're never going to work on."

This was inspired in part by Jeff Atwood, who co-founded another Fog Creek spin-off called StackExchange. "He was really opposed to tracking things, because you're just going to accumulate this list of stuff and you're not going to do it," Pryor says. "You need to fix what's broken, and if it's not broken you don't spend time on it. So from his point of view, when a bug comes in you either fix it or you don't, there is no backlog."

As developers of a bug tracking system themselves, Pryor and Spolsky obviously disagreed. But it made Spolsky think about how to create a project management system that didn't lead to an overflowing backlog. That's where Trello's other big source of inspiration, the Kanban method, comes in.

The Kanban Method

Kanban means "billboard" or "signboard" in Japanese, and it's essentially a way to visualize work flow. When used for software development, a Kanban board usually consists of several columns for different phases for a task – "soon," "in progress" and "done" for example. Tasks are written on cards or sticky notes and placed in the appropriate column. When you start a task, you'll pull it from the one column and put it in the "in progress" column. When the task is complete, you move it into the "done" column.

One big advantage of Kanban is that it's easy to see what everyone is working on. In that sense, Kanban boards—or virtual billboards like Trello—can serve as a complement to existing project management tools like FogBugz. And because it's such a visual system, it also makes it easy to see where the bottlenecks in a process are.

Kanban board. Dennis Hamilton/Flickr

Trello is at heart a virtual version of the two most important elements of Kanban, boards and cards, and it tries to retain some of the properties of those physical objects. Cards that haven't been touched in a long time even begin to start yellowing, like aging paper. And, as noted, it can be cumbersome to add a large number tasks to a Trello board. This constraint has been a boon to storage startup Box, which uses Trello across its engineering department.

"At first people complain, but it actually helps us focus on doing fewer things more completely because we can't fit so much stuff on it," says Sean Rose, a product manager for Box's platform division. "Things add up too quickly on things like a spreadsheet where you can just add lines and lines and lines."

But Trello doesn't have to be used for the Kanban process. In fact, there's no organization structure imposed by the software what so ever — it's just a place to put lots of virtual cards, and you can organize them however you'd like. Instead of tasks, you could use Trello to manage shopping lists, for example. The processes you apply can be as simple or as complex as you like. This flexibility is the other secret of the software's success, Spolsky says, comparing it to Microsoft Excel, which can also be used for a huge range of different applications.

Where the Money Goes

The basic version of Trello is free, and it essentially acts as advertising for Trello's premium versions. The idea is that individuals will start using for their own tasks, and then invite other people to join Trello to collaborate on projects and, eventually, the company they work for will end up paying for a premium account. That's exactly how Box ended up using, Rose says."One team started using it as an experiment because it's free to sign-up," he says. "Then they found out that another team was using it, then at some point we decided to just get an organizational license."

But supporting free users while waiting for revenue from premium accounts is expensive. That's why Spolsky and Pryor decided to take outside investment and spin Trello out into its own company. The company will use the money to hire more developers and expand its marketing and sales staff, with a goal of eventually reaching 100 million users.

That might sound overly ambitious, but Evernote grew from 5 million users in 2010 to 100 million earlier this year. "We're trying to track their growth. So far we're doing well," says Pryor. "We've doubled a few times. We just need to double a few more times. It does get hard along the way, but that's why we took the money."

Correction 8/1/2014 at 11:10 AM: An earlier version of this story reported that Trello raised $4.3 million. It was actually $10.3 million.