I’m sure many people have already known this. But i haven’t memorize it yet.

How to screen capture:

Shift+Command+3: Capture fullscreen.
Shift+Command+4: Capture region.
               : Press space to capture window. 
               : Hold option and drag, then
               : you will capture a region with its center in where you started dragging 
               : and one of its corners under your cursor.
Ctrl+OneOfTheTwoAbove: Capture to clipboard.
defaults write com.apple.screencapture location: Change the default location storing the images.

Apps enhancing the snapshots: Snappy and Mapture.

See more detailed here.