You pay in advance, just like you would with a pre-pay mobile phone. Whatever you pay in is held in your account permanently until used.
Your account credit is debited daily for each separate hosting account you have. Commonly these transactions will be for fractions of a penny.
You can top up instantly from your account at any time. Payments are processed by PayPal, allowing you to pay directly using any major debit or credit card, or through your own PayPal account.