PPC, or pay-per-click, are a type of ad campaign in which you pay search engines (typically) for sponsored positions in search results. You bid against other advertisers for placement in the results for individual keywords and keyphrases, and then pay the bid amount each time a visitor clicks on the ad and lands at your site.
To add to that, the more you pay for a click, the higher up you will appear in the search engines (namely, google, yahoo, and msn live). Pay per click ranges per industry i believe, i'm in the manufacturing industry and ours run from $1 per click to $15 per click, depending on how popular the keyword is. Overall it's just another way to drive relevant traffic to your site especially if you're not already one of the top organic results.
Antoher thing to understand about PPC traffic is you have to have good metrics in place to know what your bid should be. Are you trying to get people to sign-up for an email list or to purchase something? By knowing this, you can calculate how much you should bid so you aren't just watching your marketing budget go away each month.