All types of wireless plans are constructed in a similar way - they either have inclusive minutes, texts and data allowances - or they are pay as you use.
Monthly plans can either be paid in advance or post paid - the easiest way to think about it is that a prepaid monthly plan is paid in advance (say the start of that month of usage) and a post paid plan is paid at the end of the month of usage.
Paid in advance plans will generally have some restrictions - like you can't make international calls for example, this is because you've paid for service upfront. Potspaid plans are generally a little more open - so you can use things like international calling and texts or roaming as the carrier will bill you for these additional services after you have used them.
Prepay or pay as you use plans will usually involve purchasing an allowance of minutes, texts and dat - once this is used you need to top-up or re-up your balance, either with a voucher or online. These plans allow you to keep a really tight control on costs, especially if you don't use the phone that much.
The key difference in a prepay plan is that the amount you have in credit will have an expiry - generally 30 days, some carriers offer longer. This means that every 30 days you will have to top-up your. Some carriers will rollover what you haven't used, others will not - so its a use it or lose it approach.