The Monthly billing option will charge your credit card every month for the price of the plan you've selected.
The Yearly (or Annual) billing option will charge your credit card up front for 12 months of service. We give you a 20% discount in exchange for paying up front for a year. If you have any overages (extra calls, texts, faxes, etc, that exceed the allotment on your plan) during a month while on Yearly billing, we will bill your credit card that month.