The most important part of PPC is to choose your keywords those are searched by lot of people.
Suppose your keyword is “PPC” and search volume for PPC keywords is approx 12000/monthly it means approx 12000 people visit the Google first page by searching this keyword. Now if you purchased your Ad for PPC keyword it means you will get at least 12000 approx impressions for your Ad. If you have written an effective or interesting ad that gives an answer for which a user is landed on the same page, you have good chance to get click. Once a user clicks on your Ad, you charged at that time by Google. Your Ad will be live until you have sufficient payment in your PPC account.