OS X has several keyboard shortcuts (hotkeys) for copying the screen. These will save your screenshots as a jpg files on the desktop. Here's a list of them:

Cmd-Shift-3                             Take snapshot of the whole screen
Cmd-Shift-4                             Take snapshot of the selected area
Cmd-Shift-4-Spacebar                    Take picture of a window
Escape                                  Cancel
Hold Spacebar after Drawing the region  Move the selected area
Hold Option                             Resize selected area
Hold Shift                              Resize selected area horizontally 
                                        or vertically 


There's also a "Grab" application located in the Applications | Utilities folder that does the same job, plus has a few extra features such as delay.

Update

In Snow Leopard (10.6.8) the default screenshot format is png not jpg, although that can be changed to any one of many others -- including pdf -- using the free 3rd-party TinkerTool preference settings utility.

Copy colored output from Mac OS X terminal
I am trying to recover some lost images, and with all the programs I've tried, some of the photos resulted to be in a 160x120 pixel resolution. What does this mean, and is there any possibility to recover photos in original dimensions?

Any help is appreciated.
