From BlenderWiki

Screenshots

In order to facilitate teamwork and rapid prototyping, you might want to quickly take a picture of your window or entire Blender window setup.

File » Screenshot Subwindow (CtrlF3) takes a picture of your active window, and opens a “save” File Browser window, allowing you to specify the location and name of the file. As with still renders, the type of the saved picture (jpeg, png, etc.) is set by the Save Image As drop-down list, in the Format panel, Render sub-context, Scene context (F10).

File » Screenshot All (⇧ ShiftCtrlF3) takes a picture of the entire Blender window.