Ask Stewart Butterfield: serial entrepreneur, co-founder of Flickr, failed online role-playing game maker and now, chief executive of Slack, a communications start-up valued at more than $1 billion.

For a start-up like Mr. Butterfield’s, reaching that valuation in such a short time is unheard of. Slack, which offers a chat-room-like product for businesses to use as an internal communication tool, was introduced publicly only eight months ago. It is focused on the enterprise, which means it sells its products primarily to businesses, rather than individual consumers.

But with a new $120 million round of financing, announced on Friday, venture capitalists believe that Mr. Butterfield is building an enterprise company that will lead the next 100 years of online collaboration.

“It’s one of these rare things, an Internet treasure, we believe,” said John Doerr, a partner at Kleiner Perkins Byers and Caufield, a venture firm that participated in Slack’s latest round of financing, along with Google Ventures and Andreessen Horowitz. “It’s a complete Internet business stack with extraordinary user love, and that is translating into extraordinary user growth.”

At first blush, Slack sounds like many applications you probably already know. The idea is simple: Corporate teams work much better if they can easily collaborate from their computers, whether they’re in the same building, or on different continents. At its core, it springs from applications like Internet Relay Chat, or IRC, an early web tool that is a chat room at its most basic.

This is not a coincidence. Mr. Butterfield relied on IRC when he tried building another start-up, a massively multiplayer online role playing game, or MMORPG, which ended up fizzling out. His team was spread across Canada and the United States; chatting online and sharing files was crucial to the business.

When Mr. Butterfield realized his gaming start-up would not work out, the team decided to use the remaining funding to create a better version of the taped-together collaboration system his company had relied upon previously.

“We never wanted to work again without a system like the one we were building already,” Mr. Butterfield said. “It was really janky, and only incrementally functional over the years we were working together.”

Slack, then, is a much better version of what has been done many times before. (The New York Times recently started using Slack for internal collaboration.) Much like competing services Hipchat and Campfire, Slack offers chat for small, individual teams inside of big companies. It is also slickly designed and easy to use — an anomaly in enterprise software computing.

The numbers are good. Slack says it has 250,000 people using its product on a daily basis, about 70,000 of which are paid users. The company is expected to surpass $10 million in annual recurring revenue this year — a difficult feat for early-stage enterprise software companies — and is growing at a rate of $1 million in annual recurring revenue on a monthly basis, according to Mr. Butterfield. There are more than 30,000 teams using the service, sending upward of 200 million messages to one another every month.

But the real value, Mr. Butterfield said, is in the way Slack works with so many other online services. It connects to other software services like Dropbox, an online file storage service; Github, a database code management service; and ZenDesk, an online customer service platform. And everything entered into Slack — chats, Twitter messages, customer service tickets — is archived and searchable by any employee in an organization that uses the service.

This approach, Mr. Butterfield says, is exactly how online collaborators want to work. Instead of a traditional I.T. department approach, in which an administrator decides on a specific set of software for the entire company, workers can choose what programs work best for them, and use them with Slack.

The problem, however, is that this is not how I.T. managers historically handle providing software for their workplaces. I.T.’s utmost desire is usually security, and often these software models are not customizable to fit the needs of different workers.

Companies like Salesforce, one of Silicon Valley’s successful enterprise software companies, popularized a “software as a service” approach to enterprise computing, selling its software to businesses online via a subscription service. This paved the way for companies like Hipchat, Zendesk and, of course, Slack.

Now, analysts say the enterprise model of software purchasing is changing.

“Enterprises are becoming very open to these types of options, especially in the last several years,” said Rob Koplowitz, an analyst with Forrester Research. “We’ve had a mind-set change where managers want to identify what workers want, and then try to support it.”

“Purchasing decisions are shifting away from chief information officers and I.T. staff, over to heads of departments and marketing directors,” Mr. Doerr said. “What business in the world doesn’t want to empower teams?”