Just to round out the discussion a bit: your wallet contains many public keys or bitcoin addresses. For improved anonymity, it's best to never reuse an address when receiving coins. The default behaviour is to choose another bitcoin address as soon as you've received payment on the current one.
Now, you may not care about anonymity, but many people do, and it's best for those people if as many people as possible use the anonymity features as much as possible. Think of it this way: if only those people who cared about anonymity put their mail (regular, not e-) in an envelope, but everyone else just wrote on a post card, then the envelopes would stick out like a sore thumb.