Should I prefer that people link to my site with “www”, or without?
By default, our servers respond identically for Web site visitors who use “http://www.example.com/” or “http://example.com/” — that is, our servers don’t care whether the “www.” is present.
However, many customers want to “canonicalize” the exact hostname they use in URLs and marketing materials, making sure that everything uses the same format.
If you do this, we strongly recommend using “www.example.com”, not “example.com”. Doing so will avoid potential problems in the future.
The exact details are pretty technical, but DNS changes, for example, have far more flexibility if you include an extra level. It allows you to make changes to how your Web site is handled without worrying that the same “example.com” DNS change will affect something else like e-mail.
In particular, using “www.example.com” allows you to change to either A CNAME record or an A record in the future. If you use only “example.com”, you won’t be able to use a CNAME record, which limits your ability to use certain other services on the Internet.