dd
in command mode (after pressing escape) will cut the line, p
in command mode will paste.
Update:
For a bonus, d
and then a movement will cut the equivalent of that movement, so dw
will cut a word, d<down-arrow>
will cut this line and the line below, d50w
will cut 50 words.
yy
is copy line, and works like dd
.
D
cuts from cursor to end of line.
If you've used v
(visual mode), you should try V
(visual line mode) and <ctrl>v
(visual block mode).