Isn't less just more?

The difference is mostly historical at this point, I believe some systems even have more and less hardlinked to the same binary.

Originally, more pretty much only allowed you to move forward in a file, but was pretty decent for buffering output.

less was written as an improved more that allowed you to scroll around the displayed text

The first line of my man less pretty much sums it up:

Less is a program similar to more, but which allows backward 
movement in the file as well as forward movement. 

less is a lot more than more, for instance you have a lot more functionality:

g: go top of the file
G: go bottom of the file
/: search forward
?: search backward
N: show line number
: goto line
F: similar to tail -f, stop with ctrl+c
S: split lines

And I don't remember more ;-)

In addition to the extra features already mentioned in less, it also has a v command which opens the current file in vi at the same position. When you exit vi, you will be back in less with the (possibly updated) file shown.