Who he is: Current CEO of Twitter, succeeding Evan Williams. Costolo is a fairly public figure, though not as famous as Jack Dorsey—and this is interesting, given that when he left college with a degree in Computing and Communications Science he's said to have ignored offers from tech firms in order to pursue a career in improv comedy.
He then worked as a consultant, cofounded a web development firm, a web page monitoring firm, and worked for Google when it bought his company FeedBurner. He left Google in July 2009, and joined Twitter in September of that year as COO. Costolo's management of the company has been lauded, and he's active on Twitter (famed for some pithy put-downs). At Fast Company we called him the "Confucius of Tech" for his evident wisdom and the ease with which he explains the future of Twitter. For example, as a conversational second screen to TV.
His relationship with Twitter: He took over temporarily as CEO in October 2010, and the post became permanent after then-CEO Evan Williams's paternity leave was up.
What he's worth now: Unknown, though he is certainly well off, having sold a number of successful companies, including the FeedBurner-Google deal. In 2011 his net worth was estimated at $150 million. More recent estimates are closer to $1.5 billion.
What he’ll earn via Twitter’s IPO: Costolo owns 7,589,608 Twitter shares. We can guess from the end of first day trading that at $44.90 a share Dick is now worth something like an additional $340 million. Though that can go up, or down of course.
What he’ll likely do with the money: Costolo shows no sign of leaving Twitter. He has been a serial entrepreneur, though, so it's possible he may use the money to leverage future startups. Previously he'd been appointed to President Obama's National Security Telecommunications Advisory Committee, but whether this indicates Costolo may have a future interest in politics is anyone's guess. Given how he managed a recent ruckus about representation of women at Twitter...perhaps not.