i have never seen an ad on youtube. wtf are you guys watching that has ads?
Loads of videos have ads.
Youtube users, if they don't get any copyright warnings, and if they upload several modestly popular videos, will be given the option of enabling ads for their videos, and receive a cut of the income.
You don't need much. I had about 20 videos, with about 10k views between them, when I got invited.
It used to be the case that the only ad option was small overlays at the bottom of the screen. Youtube then added pre-roll video ads as an option. In the last few months, they've made pre-roll ads the default. I usually turn them off on new videos, as they are annoying, but sometimes I forget. Generally, user ads are skippable. Even so, the income generated from pre-roll ads is MUCH higher than banner ads, so a lot of users like to keep them on.
Some corporate users, get extra options, and they can specify unskippable ads, or intermission ads.
These same corporate users, can also apply ads automatically to user videos which contain their content. So, if I were to upload a copyrighted music video owned by a major publisher, it would have their choice of ads, which would usually be unskippable, because of the greater revenue.