screen the terminal multiplexer.

To split vertically: ctrl-a then |.
To split horizontally: ctrl-a then S (uppercase 's').
To unsplit: ctrl-a then Q (uppercase 'q').
To switch from one to the other: ctrl-a then tab

Note: After splitting, you need to go into the new region and start a new session via ctrl-a then c before you can use that area.

Basic screen usage:

New terminal: ctrl-a then c.
Next terminal: ctrl-a then space.
Previous terminal: ctrl-a then backspace.
N'th terminal ctrl-a then [n]. (works for n∈{0,1…9})
Switch between terminals using list: ctrl-a then " (useful when more than 10 terminals)
Send ctrla to the underlying terminal ctrl-a then a.