A Bitcoin address is just an identifier (cryptographic hash) of a public key in a keypair. For every transaction you do, you can create a new one. It's also possible to publish one publicly, as an address to anonymously send donations to, which can also make it easier for the public to track how many donations have gone to that address, but for most purposes, you create one per transaction, so you can keep track of how much has been spent in that transaction.
Any time you send Bitcoins, the way it works is that it sends the portion you specify to the given address, and generates a new keypair for yourself to send the change back to an address you control.
So, unless you just happen to leave a single address published publicly as your donation address, you don't have a permanent address. If you want to collect any additional information associated with a transaction; an email address to send thanks (or hit up for donations later), information to collect for tax purposes, etc, you just create a new keypair and thus new address for that transaction.