It's hard to recall life before YouTube, which was born April 23, 2005. That's four years in real time, but just a few blinks at Internet speed. The video-sharing site clicked open sweeping changes in pop culture, politics, news delivery and business. It spawned other services such as Hulu, Google Videos, Yahoo! Videos, filmmaker sites and more -- including a video-only channel launched last week by PBS, the public TV network.

These are the latest evolutionary changes in how we watch TV -- a shift that began in the late 1970s when video cassette recorders (VCRs to your teachers and parents) became common in homes. Cable services later introduced video-on-demand channels. Next, consumers bought DVD recorders and this decade began watching online videos. Broadcast and cable networks now support that migration by posting full episodes of shows on their own sites or at Hulu.com, owned by the parent company of CBS. Next up: iPhone applications.
It's not all about entertainment, sports or home videos. Educational sites show university lectures, a Periodic Table of Videos (periodicvideos.com) with short clips on each chemical element, and more than 200 speakers in the popular TED lecture series about Technology, Entertainment, Design (ted.com). The presentation below by a Los Angeles technology executive begins with two students telling him why they prefer videos to TV.

YouTube is hustling to keep pace by vastly expanding its library of full-length movies and TV shows. A dozen new content partners include film studios and Universal Music Group. (Warner Brothers and Sony BMG came aboard earlier to share music videos.)
And this month, it's virtually impossible to miss YouTube's digital fingerprints all over the news. Tens of millions of people have watched Susan Boyle, the Scottish singing sensation on "Britain's Got Talent." The president of Domino's Pizza posted an apology for a prank video put up by two ex-employees. Music critics across the country weighed in on the debut of the YouTube Symphony Orchestra at Carnegie Hall in New York.

YouTube says: "A couple of guys with a dream pulled together and created a site that is brilliant in its simplicity -- all you want to do is watch clips. From there, it became this global phenomenon." -- Chris Dale, company spokesman

Cable executive says: "All channels delivered by cable, satellite and telecom should be available on demand on your television set and they should be available on demand on your PC and your mobile-broadband device." -- Jeff Bewkes, Time Warner chairman/CEO

Blogger says: "These [video] technologies are game-changers. They are drastically and inevitably changing the shape and dynamics of our world and lives. . . . When you think of YouTube, remember that there's a lot more than cute kitties involved." -- Lauren Weinstein, Los Angeles blogger and Wired News columnist