The Print screen key will take a snapshot of your entire screen. If you are using multiple monitors it will capture all of them as your entire screen is all the monitors.
I have recently used "Camtasia" ...it is super... Camtasia Studio 2, free trial available...do a search... Don
for years I used PrtSc but recently someone pointed out MWsnap (http://www.mirekw.com/index.html). Has no ads and is freeware, you may give a dontating. Highly recommened.
It's ok if it's the whole screen and you don't have to archive in some specific directory each day. But if it's a portion of the screen and several screen copies per day to archivate by date for example it can become cumbersome. After several solutions, the best I found at the moment is to combine screenhunter with cyclops2: you can use printscreen with screenhunter (or analogx above) and cyclops will popup to enter a filename and save into a predefined directory you can change easily (that's why I like it I have to change directory often).
Grabzilla does work well, free, and I like it. Is there any way though to adjust the resolution of the saved screen image? When I "grab" charts, it saves images at 250-400kb sized jpeg images, they look a bit low to examine as high quality archives (some of my chart studies have small details I wish to view easily). If I could bump up the image quality I would really be happy with it!