Mac OSX has multiple ways to take a screenshot depending on what you need to capture.

Entire Screen Region (crosshairs)
Save to Desktop CMD+SHIFT+3 CMD+SHIFT+4
Save to Clipboard CMD+CONTROL+SHIFT+3 CMD+CONTROL+SHIFT+4



Detailed Explanation

CMD+SHIFT+3 will take a screenshot of the entire screen and save the image to the desktop as a png file.

CMD+CONTROL+SHIFT+3 will copy the entire screen to the clipboard so it can be pasted somewhere else.

CMD+SHIFT+4 will turn the cursor into crosshairs and allow you to take a screenshot of the selected portion of the screen. After clicking the screenshot is saved to the desktop as a png file.

CMD+CONTROL+SHIFT+4 will allow you to take a screenshot of a portion of the screen and the image will be saved to the clipboard.

CMD+SHIFT+4 then SPACEBAR will let you take a screenshot of a specific window and save it to desktop.

CMD+CONTROL+SHIFT+4 then SPACEBAR will let you take a screenshot of a specific window and save it to the clipboard.

Using Grab

Mac OSX comes with a utility called grab. This is found under applications > utilities > grab. It is a screen capturing program which can be used to achieve the same things above and more.