I know these are very basic questions, but I didn't see these specific questions addressed anywhere on the Adwords tutorial/help pages.
You are charged on a monthly basis.
This depends on the payment option you selected (which is determined by what country you are in).
For most people who use credit card payment options, you're given a line of credit - depending on how quickly you exhaust that line of credit depends on how often you are charged.
If you don't spend enough to meet the line of credit, Google usually charges you at the beginning of the month.
[adwords.google.com...] has many of the billing answers you're looking for.