I need to make one correction to what I wrote above. In my haste, I accidentally called your Bitcoin address your "public key". Actually, your public key is a related but different concept. Properly said, the string that starts with "1" is just a Bitcoin address.

Anyway, just wanted to correct myself that those are two different but related concepts. This has no impact on your use of Bitcoins -- just use the one that starts with "1" to give out to receive Bitcoins. Most programs won't even show you the public key I'm talking about, so no need to worry about it.