With the AdWords postpay option, you pay only for the clicks (or for some placement-targeted campaigns, the impressions) you've already received, so you're not billed in advance. Google bills you after 30 days or when your account spend reaches your billing threshold, whichever comes first.
Billing Thresholds Explained
When you create and activate your Google AdWords account, your initial billing threshold is set at US$50. The billing threshold is raised in increments each time your account spend hits the threshold before 30 days have ended. The initial billing threshold of US$50 is first raised to US$200, then to US$350, and then to the final amount of US$500. The amount billed may be slightly in excess of the threshold if the account accrues costs very quickly.
Note that a small, one-time activation fee applies when you create and activate your Google AdWords account. See the activation fee in your currency. 
Example
Let's say that advertiser Alan opens an AdWords account and starts accruing costs on Oct. 1:
- On Oct. 5, Alan reaches his initial billing threshold of US$50. At that time, we charge him for the amount accrued (which might be in slight excess of the billing threshold if he accrued costs quickly). Alan's billing threshold is increased to US$200. A new billing cycle begins.
- On Oct. 25, Alan reaches his new billing threshold of US$200, triggering AdWords to charge him for costs accrued. Google again raises his billing threshold to US$350, and initiates a new billing cycle.
- On Nov. 18, Alan accrues more than US$350 in charges. Google bills him and raises his billing threshold to US$500.
- Alan's next bill will be processed on Dec. 18 or when he exceeds the final US$500 billing threshold - whichever comes first.
Learn about when you're billed if you prepay for your advertising costs.
