Cmd + shift + 4
The mouse will then turn into a cursor allowing you to select an area of the screen to take the screen shot of. Clearly an incredibly useful feature particularly if you're looking to produce documents (or blogs!) which include lots of helpful screen shots!
The problem is that if, like me, you have a locked desktop (and for the life of me I can't work out how it happened; but I like it and don't want to unlock it!) the first time you go to use this you get the following error;
(Just a little note for Apple - telling me where it's trying to save the screen shot would have been helpful, as would offering me the change to save it somewhere else!)
The problem is that the screen capture utility is set to save the screen shot to your Desktop and to fix this you have essentially two options; change the location it's trying to save to to somewhere it can access (my preferred one!) or to unlock your desktop (a lot easier!)
To change the location you need to use Terminal. Bring up a terminal session (Cmd + Space > Terminal or go to the top right and click on the search and enter Terminal);
Copy/ Paste the following two lines (you'll notice your bars will refresh);
defaults write com.apple.screencapture location ~/Downloads/
Close terminal and try again, the screen shot should now appear in your downloads folder (if you want it in Pictures just replace Downloads with Pictures in the first command above).
I'll blog unlocking your desktop separately.