How does a bitcoin address appear?
Bitcoin address, or simply address, is an identifier containing about 33 alphanumeric characters. In the current version of the protocol it starts with 1 or 3, using this identifier you can perform operations with bitcoins.
Technically, a bitcoin address is a 160-bit hash from the ECDSA public key of the key pair. Each newly generated key pair is assigned an address for receiving coins. Bitcoin addresses (public keys) and their corresponding private keys are stored in the file wallet.dat. This file belongs only to you and you need to constantly make backup copies of it. Bitcoin Wallet does it automatically for you.
Bitcoin system allows you to generate as many addresses as you wish. And each of them will be completely independent. There is no main address. The application only for your convenience will automatically change them from time to time to increase your anonymity in the system. In this case, all your previous addresses will remain working forever.