PPC, or Pay Per Click, is an advertising solution that allows you to run ads on the search engines and on websites. Every time someone clicks on one of your ads you are charged for that click.
The most well know and widely used platform for this is Google AdWords. With an AdWords account you can bid on keywords and create ads that will show in Google's Sponsored Links on their Search Results Pages (top left and down the right hand column).
Pay Per Click works on the simple model of the more you pay and the higher quality your PPC account is the higher your ads will show and the more they will show throughout a given period of time.
Cons of PPC are that it can be expensive and the ads stop running when you stop paying.
Pros of PPC are that it is the fastest and most immediate way to get traffic to your site directly from an ad campaign. In addition, PPC allows you to gather useful data about which keywords and phrases relevant to your site convert the best and provide the highest possible ROI.
I highly recommend that you do a little research, via AdWords, to learn about the platform before throwing any money at an account as PPC is one of the fastest ways to burn money if not setup correctly. There are tons of companies that can manage your PPC account for you, but I suggest being very cautious with most of these as too many of them take advantage of unknowing customers. Just do some research and you will be fine...
For more information check out these links:
http://www.realestatemarketingblog.org/paid-search-advertising/ (real estate related but still a good, simple explanation for anyone)