Every time you send money, a new address is created to receive the "change". (Not quite change, but close enough to call it that). If you receive 100BTC from someone then restore your old wallet, you'll still have the 100BTC. If you then send someone 1BTC and restore your old wallet, all your money will be gone -- the 99BTC "change" will have been put into an invisible address which is only present in your new wallet.
There are coin-control forks of bitcoin for avoiding this, but if you don't know how to use them you can seriously compromise the anonymity of your transactions.

Edit - Re the "recycling" of addresses/wallets: Wallets are never "registered" with other clients. You can send money to a wallet that doesn't exist, then if you're the luckiest guy who ever lived, generate that wallet a year later and find the funds you sent (back when it didn't exist) already in it. The chances of you generating it though is small enough to be considered impossible.
The flipside to this coin is that wallets never need to be unregistered either. Simply not using an address is enough to 'recycle' it.