Ctrl-w s        horizontal split same file
Ctrl-w v        vertical split same file

:sp filename    horizontal split with other file
:vsp filename   vertical split with other file

Ctrl-w w        Cycle through the open windows
Ctrl-w arrow    Move between windows.  (also Ctrl-w [hjkl] work)

Ctrl-w +        Increase window
Ctrl-w -        Decrease window   (or use the mouse)
Ctrl-w =        Reset windows (equalize them)
Ctrl-w _        Maximize current window vertically
Ctrl-w |        Maximize current window horizontally

Ctrl-w r        Rotate windows
Ctrl-w R        Rotate windows in the other direction
Ctrl-w [HJKL]   Move windows

Ctrl-w c        Close window
:q              Close current window
:only           Close every window except the current one

:help ctrl-w

Working with windows