Very useful to open multiple files on the same screen or the same file at different places. Splits consists of sharing the vim's screen.
There are basically two possibilities :
$ vim -o foo bar = open foo and bar in split $ vim -O foo bar = open foo and bar in vsplit
'^' is the CTRL key.
^w s = share the screen horizontally in two ^w v = share the screen vertically in two
^w j = select the bottom split ^w k = select the top split ^w + = extend the current split a line ^w - = reduce the current split a line
^w h = select the vsplit on the left ^w l = select the vsplit on the right ^w > = extend the current split a column ^w < = reduce the current split a column ^w w = switch between splits ^w [up,down,right,left] = select the split
^w H = move on the left ^w L = move on the right ^w K = move on the top ^w J = move on the bottom
^w q = close the current split ^w o = close all the splits but the current