Cartoons randomly remixed together would be the most simple definition. Some of them are well made, have make some kind of sense and some kind of clever joke, but that's the 1% of youtube poops. The remaining 99% is a monument to the amount of stupid people all over youtube: They consist of taking an already popular clip from a cartoon, picking one of the frames and lagging it for a while so it will sound like stuttering, repeating the clip several times applying the same old video effects from wmm we have all seen so many times already and mixing up the words and sentences with other cartoon clips to make it sound like they are saying something silly like "Your ass drives me crazy"
For some reason, it usually results in most people with a retarded sense of humor loving them and making them more popular, eventually gaining enough fans to brainwash people who didn't have that sense of humor in the first place to make them think it's actually funny.
Random youtube poop fan: ROFLMAO!!11! hes sayin I wan some mama luigi in slow motion now! lol! nao its sayin in hig speed! its sound so funy! zomg teh colors are all liek changin now!