On Thursday, Twitter announced that it submitted paperwork to the Securities and Exchange Commission for an initial public offering: 

As Quartz explained earlier this year, the confidential filing takes advantage of some relatively new SEC rules that allows companies with annual revenues of under $1 billion to keep their filings secret until just three weeks before starting to market shares. They continue: 

It was largely proposed and pushed by venture capitalists in Silicon Valley, who said it would stem a decline in US IPOs by reducing the burdens of going public felt by smaller firms. Ordinarily, a company has to make public a lengthy discussion of its financials, strategy, and risks months before an IPO, giving investors—and competitors—more of a chance to evaluate the stock.

Twitter, recalling the circus-like atmosphere (and disaster) of Facebook's IPO, has previously said that it wanted a "low-profile" offering, rumored to hit some time near the beginning of 2014 . As of last February, the company was valued at about $10 billion by one of its investors (we have a breakdown of that valuation here). 

According to Bloomberg News, Goldman Sachs will be the lead underwriter for the IPO. Sachs, incidentally, was one of a handful of companies sued by Facebook investors in May of 2012. Investors accused the firms of misleading them over Facebook's revenue estimates. 

Twitter CEO Dick Costolo heightened rumors of an impending filing when he declined to take questions at this week's TechCrunch Disrupt conference after a brief speech, despite expectations that the company would file any day now.