Correction in Question : ... is the DIFFERENCE ... Pre-paid is when you pay in advance for a service that you are going to use/avail. Post-paid is when you pay for a service after you have used it or availed it. e.g. A pre-paid mobile connection requires you to recharge it with some money in order to use it and without recharging and with zero balance, you find yourself unable to make calls. A post-paid mobile connection allows you to make calls and pay the monthly bill at the end of each month.

When people prepay, they pay in advance of using a service, but when they postpay, they pay after having used a service.

Pre = before; post = after. Pretty much!