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).