Their humor is silly and a bit surreal, decorated with various fake mustaches (plastic horses, magic marker, pipe...) and often featuring increasingly violent slapstick fights with bats and frying pans and cartoonish sound effects. They're an acquired taste, or they're tailor-made for the 13 year-old boy who likes jokes about passing wind. One or the other.
Mortimer met Reeves (real name Jim Moir) at one of the latter's stand-up shows, after which he approached him and found they had the same sense of humor, though it's really more than that -- the rapport of a double act is its triumph or downfall, and Vic and Bob demonstrate some of the best. You couldn't replicate the banter they do because you're not on the same wavelength. They're on a different plane completely. A plane where Ulrika Jonsson is a stewardess in a short skirt and Matt Lucas is the baby pilot in a onesie.
They probably don't consider themselves surrealists. I'm pretty sure they just do what they think is funny. This results in absurd, outlandish sketches and characters. Some are visually grotesque, adding to the surrealism of the show.
I guess what I find hard to write about is why I find them so funny, why I was compelled to watch this show, and Bang Bang It's Reeves and Mortimer, and the abysmal Randall and Hopkirk (Deceased), and more than four series of the celebrity-style quiz show Shooting Stars. I think maybe it's because I adore the pair so much. I love their tremendous chemistry. It's a joy to see them interact. I also love the surreal element, intended as such or not. The Mighty Boosh certainly must have been influenced by these two.
Really good stuff. Juvenile and strange, but good. I don't know what else to say!