As the name suggests, the difference between Prepaid and Postpaid plans is about when you pay for them.
Prepaid plans work by having you 'recharge' your account before you can use it. Recharges comes in a range of prices with varying inclusions, and you are free to switch which recharge option you choose every time you top-up.
If you use up all of your credit on a Prepaid plan, you need to recharge your account before you can use these service again.
Postpaid plans are more like an agreement. You choose a plan and every month your telco will bill you and automatically refresh your plan inclusions on the first day of your billing cycle.
If you use up all of your credit on a Postpaid account, your telco will let you continue to use its services for an extra fee, which is added to your bill total in the next month.
Both plan types have pros and cons, but if you are really concerned about sticking to a budget, then a Prepaid plan is the best choice.