To quote the manual, less is the opposite of more. To be precise, it is a file pager of UN*X systems. Originally it was named because it could page backwards through files (or even pipes, if set up to buffer them!), although nowadays more also does that.
less has zillions of commands. It is also highly configurable: you can set up the appearance of the prompt (and the details it includes), default option settings, keybindings, and an input preprocessor (so you can view gzipped files directly, or tarred directories, or anything else).
It's all too complex for my feeble mind; I usually prefer more.