What? A giant front page ad, one to two pre-roll ads (with accompanying banner ad in the sidebar), semitransparent text ads on the bottom of the video, and another video ad every 4-10 minutes in the video.

Personally I think they are great at finding interesting and relevant ads, which is why I don't block the ads. Howerver I hear the experience varies.

YT does not show ads if you have adblock. Adblock cannot actually block them, but google detects it and chooses to not show them (the idea is that if you have adblock, ads are unlikely to be worth showing to you)