PPC stands for "Pay Per Click." The phrase is often interchangeable with the phrase "Cost Per Click."
PPC is one of the popular methods used to conduct and track in Search Engine Marketing campaigns. With Pay Per Click Marketing, advertisers only pay for every time someone clicks on their add instead of paying every time the ad appears on the page.
PPC is a very popular method of tracking search engine marketing performance as it helps give data as to exactly what keywords people use when they click the ad. With advanced tracking and use of Search Engine Marketing platform tools such as AdWords, PPC campaigns show which keywords best convert in campaigns - "convert" meaning "performing desired action." Such actions typically mean capturing contact information or sales.
Data captured in PPC campaigns are often used in a web project's SEO, using the best converting keywords as target goals for the Search Engine Optimization campaigns. The reason this is done is because the keyword data has already been shown to work.
Here's a Good Google Video Tutorial about their PPC Search Engine Marketing Platform - AdWords