1. In an Apple computer you press command+shift+4 , all at the same time. Your mouse will change from the usual pointer to a different type of cursor; it looks like a miniature graph for a math function.
2. Using this new cursor, start copying the figure you want to capture on your screen by clicking and holding any corner of the figure. Once this is done, proceed to cover the entire figure, which will cover the entire surface with what looks like a gray rectangle. This gray rectangle contains what will be captured. Finish the capture by "unclicking" when the desired figure is covered entirely by this gray rectangle.
3. Once you have chosen the area you want to capture, the picture will be sent automatically to the desktop folder. In order to access this screenshot for future use, it can be manipulated on the desktop as if it were any old file.
For example, if you wanted to include the picture in a document or website, its location would be "Desktop".
If, maybe, you wanted to move the photo to a folder, where you have all your others screenshots, you would simply drag and drop it to the folder of your choice.
This is a screenshot of a kitten. I hope it made you smile =D