I was in need of capturing the current window (for the last post actually) to store as an image. I went to press Alt + PrnScrn. Ah. Where's the print screen? So after doing a bit of research, here are all the ways you can do a screen capture in OSX.
Press Command + Shift + 3
This will place a document in pdf format on the desktop. You can then open and Save As...
Capture selected Window
Press Command + Shift + 4 + Space. This will turn the cursor into a camera and you then click the window you want to capture. Again it will save to the desktop in PDF
Select Area to Capture
Press Command + Shift + 4. This will turn the cursor into cross hairs which you then select what you want captured.
While these are useful, what if you want to save to the clipboard instead of saving to the desktop? Well Apple has thought of that too - just press the Cntrl key with all the combos above.