command-shift-3 captures the whole screen as a TIFF file in Mac OS X 10.0.0 through 10.1.5, and in 10.2.x as a PDF.
On later operating systems, it creates a PNG file. Apple's Preview application can export the PDF or PNG file to JPG, TIFF, or PDF. JPG typically is the smallest file format, and easiest to post to Apple Support Communities.
The file will appear on the desktop of the computer, or Users -> yourname -> Desktop folder on the boot hard drive. By default the Preview application will open these documents and offers an Export function in its File menu and make it able to export into several possible image formats. If Preview does not open the screen capture from the Finder, open Preview from the Applications folder.
command-shift-4 will give you a crosshair which you can click and drag drag to the opposite corner of a rectangle and let go to capture the rectangle. If you want to capture a specific window, menu, menubar, dock, instead of clicking, click on the spacebar and move the mouse pointer over the area you wish to capture. It will hilight and with another click capture it. These documents like the command-shift-3 will open with Preview by default.
The Applications -> Utilities -> Grab offers the ability to capture timed capture screen shots.
Additional information about command-shift4- capabilities are on Apple knowledgebase article below:
If running windows control-alt-print screen followed by Edit menu -> Paste in the image editor in your Windows Accessories folder, or using the Snipping Tool (available on Windows 7 and later) lets you make a screen capture. Be sure to save as JPEG before posting the image here.