.Mac is Apple's online service. It offers net based storage in various sizes, email accounts and net access to them, syncing ability between several Macs (if you are lucky enough to own more than one), iWeb integration and a few other goodies. It is priced at around $100.00 per year. Frankly, I have never found it to be worth this price, but many have. It is a personal preference item in the end.
You can find many of its components available for free, or at lower prices if not free, out there on the web - free storage, free email... still, none of them feature the overall integration with the total Mac experience that .Mac does. Read the above link, evaluate it, and decided if it is worth $100 a year to you!