Wireless service plans give you a certain number of airtime minutes to use each month. You'll pay a per-minute fee for any calls that you make over this limit, and most plans won't let you carry over leftover minutes to the next month. With cell phones, you pay for the calls you receive as well as the calls you make, so you will need to figure incoming calls in your allotted airtime minutes.
Minutes also come in different varieties.
Most popular plans give you minutes that you can use at any time of day. These are conveniently referred to as anytime minutes. You'll also see companies still selling plans with blocks of airtime divided up into peak and off-peak minutes. Peak minutes come at a premium and can be used from 7 a.m. to 7 p.m. on weekdays. There aren't very many plans that restrict your calling by time of day anymore.