Every time you use a card a 'shadow' is put on the account - the money is still in your account but has been allocated elsewhere and you can't spend it.
When you check the balance it says, for example:
Balance: �100
Available balance: �80
When the money is in the vendor's account, the shadow is removed.
I do not that vendors are twitchy about 'cardholder not present' transactions when the goods are going to an address that is different to the one registered on the card. This is simply because if the cardholder tells their credit card company the transaction is fraudulent, it is whipped straight back off the vendor and given back to the card holder.