If you're happy always to take a picture of the whole screen, then Print Screen is fine. But, if you just want to get a particular section of the screen, or one particular window, then you'll need additional tools.
Jing is great for quick and dirty screen shots, and 5 minutes videos. It's also free.
Snagit is what I use when I want to manipulate the images after capture. It's not as full-featured as a dedicated image editor, but is designed to help you get the most out of screen capture images. It's currently £38.95 - but there is a free trial.
I'd recommend both.
A simple tip that you might not be aware of....
As you know [PRINT SCREEN] creates an image of whatever is on your screen. However, pressing [ALT] + [PRINT SCREEN] will only create an image of the window that you are currently focused on. It also means that all your task bar items won't appear in the image.
Kristina et al.,
Microsoft Office 2010 has a screen capture and screen clipping facility in all of the main applications (Word, Excel, PowerPoint, Outlook). It's on the Insert tab. As I work with Office on a daily basis this has been a real time-saver as I don't need to use another piece of software and the image is put straight into where I want it.
OneNote has had this feature for a couple of versions now. OneNote 2010 has the additional functionality of being able to extract text from an image. Not perfect but goes quite a way.
Another vote here for Snagit, I can't fault it, it is feature rich and really easy to use.
Windows 7 has a built in screen capture tool, called 'Snipping Tool', which allows you to take normal screenshots, or freeform (hand-drawn) screenshots, you can then email, annotate, or save the image as JPG/PNG/GIF etc.
As well as the tools that others have suggested, you might like to try the following two.
Irfanview www.irfanview.com It's free. One of the advantages it has over just a simple Windows screen shot is that you have an option to include or not include the mouse pointer, which is very useful when you want to use a screen shot say to illustrate instructions on selecting something from a menu. (Irfanview does more than just screenshots, but that feature is quite easy to use and well implemented.)
FastStone Image Viewer is another useful free program, www.faststone.org It would be good for adding labels to your screenshot image, such as an arrow and a label saying "Click here" or whatever. (The same people also do a screen capture program called FastStone Capture, but it's shareware, not free and costs $19.95. Good though. But there's no reason why you shouldn't capture what you want with IrfanView and then add labels to it with FastStone Image Viewer.)
I like GIMP, which someone suggested, but I wouldn't use it for this. It's a powerful program, but there would be quite a steep learning curve for the sake of one simple function.
I've used Jing in the past (and it was great) but now I rely on the Snipping tool that comes standard with Windows - I'm so glad someone told me it was there (just search for Snipping in the Start search box).