Are you just wanting to take snapshots of your desktop or are you looking to record the desktop as you perform functions? Also, what OS are you using? If your using windows 7, there is a built in recorder for recording the desktop, just type PSR into the search field and it will launch.
If all you need is to take a snapshot when you want one, press the <Print Screen> button on the keyboard and it will capture your current desktop (The whole desktop), then paste it into whatever graphic program you want to use to crop out stuff you don't need.
Also, if you hold the <ALT> key as you press the <Print Screen" button, it will only capture the open application, so you can size it on the screen first, then snapshot it.
There are tons of screen capture utilities out there. Hit downloads.com and read through them and find one that has features you are looking for.