Shift-Command-3 = Capture the screen to a file
Shift-Control-Command-3 = Capture the screen to the Clipboard
Shift-Command-4 = Capture a selection to a file
Shift-Control-Command-4 = Capture a selection to the Clipboard
I use Grab.app and the little elephant menu option from Evernote. Both work well for me. I tried the SnagIt beta and that worked well too. Those are options.
You can change the filetype using the following commands in terminal.
defaults write com.apple.screencapture type png
defaults write com.apple.screencapture type pdf
defaults write com.apple.screencapture type jpg
defaults write com.apple.screencapture type tif
defaults write com.apple.screencapture type psd