Taking screenshot in Windows is easy because traditional keyboard has a key at the end of function key row for "print screen". Press it and paste the image copied onto a default program/application like "paint".
In OSX you got to know below key symbols first
Note that in MacBook keyboard layout Control key behaves not the same as the control key in a Windows keyboard, e.g. to add a new tab in your broswer you need "command + t" not "ctrl + t" and copy is "commend + c" not "ctrl + c".
To look up what keyboard shortcuts are available to you, you click the blue apple on the top menu bar > system preferences > keyboard & mouse > keyboard shortcut.
To save a screenshot to file on desktop it's simply "command + shift + 3". To save selected area of screen to file it's "commend + shift + 4" then drag the corner. The file is saved as png then you could use the default application "Preview" to save the file as other format.
What I wanted to do was to paste below image here. iStat Pro (you see just a small section of it below) is a nicely designed widget which gives you lots of information about your Mac. This is far better than any sidebar gadgets available to Vista.