A Bitcoin address, or simply address, is an identifier of approximately 33 alphanumeric characters, always beginning with the number 1, that represents a possible destination for a Bitcoin payment. Addresses can be generated at no cost by any user of Bitcoin. For example, using the Bitcoin software, one can click "Get Address" and be assigned an address. An example of a Bitcoin address is 175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W.
Like e-mail, you can send Bitcoins to a person by sending Bitcoins to their address. Somewhat unlike e-mail, one person can have an unlimited number of different Bitcoin addresses. A unique address for each transaction is recommended as a best practice, and enhances anonymity when using the system.
Creating addresses can be done without any contact with the Bitcoin network.
For every visible address, there is a corresponding secret number known as a private key. When using a Bitcoin client, private key numbers are typically stored in the wallet file. The private key has a special purpose - it is mathematically needed to create transactions acceptable to the Bitcoin network that respend the funds originally sent to the address. If the private key to an address is lost (for example, in a hard drive crash and the user didn't have backups of the wallet file), any associated Bitcoins are lost forever.