How many megapixels should your digital camera be capable of capturing if you want to print a postcard size (4" x 6") picture? To print it at 300 dpi, the standard resolution, you'll need 300 dots or pixels for every inch. That is 4 x 300 x 6 x 300 = 2,160,000 pixels. That's 2 megapixels.
It's as simple as that, but many people are confused with words like dpi and megapixels, and even pixels (picture cells), in fact. If you'd like a quick reference for the relationship between megapixels and printed pictures without dealing with multiplications and technicalities, visit http://www.design215.com/toolbox/megapixels.php
There's a colourful representation there that any digicam user should find handy.