||The basic online payments system is set to use PayPal as the payment provider, so you'll need to set up an account with them.
The payer visits a form on your website, and enters a reference number you have provided, and the amount they are paying. It then passes them to PayPal to make payment securely, PayPal send back a little background feed and the script processes this, if everything is okay, i.e. payment went through okay, it sends them a courtesy email - Dear Customer X, thank you for your payment of ?X on order reference XXX.
It will also send you a carbon copy of the email so you know who paid how much on what order number.
If the transaction was aborted or their card was rejected, it sends them an error email and sends you a carbon copy of it.
It's all very simple, but it gives out the instant receipt which is required and lets you know within minutes of being paid.
Very occasionally, when PayPal are very busy, there can be a delay of up to 20 minutes between payment and the emails arriving, but I've only noticed this once.