The videos are organized into channels by human editors, and they're pulling from video content that had been scattered around Yahoo's sites. Screen has content from ABC, CBS, FOX, NBC, FX, Discovery, and other TV networks. Not all of the shows are available as full episodes, and some link out to external sites—including Hulu—for now.
ABC shows will probably be the most readily-available on Screen for now, since the company announced yesterday that it would stream its content to Yahoo, as well as combine news resources. Yahoo also announced some new original programming aimed at women, in three-minute segments.
Yahoo claims that with original and licensed content taken into account, it's the largest video provider on the web. Yahoo is pretty massive, but until now it's been mostly an anachronistic mess of architecture. Yahoo Screen's an effort to clean that up into something usable, and it's certainly better than the state of things before. But taking an early look at it, it seems more likely to attract (and keep) the millions of legacy Yahoo users than pull traffic from other established streaming hubs like Hulu. [Fast Company, Yahoo Screen Beta]