PPC stands for Pay Per Click advertising. Pay Per Click is a form of advertising in which you pay each time someone clicks on your banner or ad.
If you're familiar with Google, you've probably seen that when you receive search results there are often ads at the top (in a yellow box) and to the right of the page. Advertisers bid on particular keywords so that when someone searches for let's say "car insurance," their ad will show.
If your looking to get into PPC you can go through Google Adwords, Yahoo! Search Marketing, or Microsoft Ad Center (Bing). There are many things to consider when going the PPC route such as the competitiveness of the keywords you're bidding on. For instance, I've seen phrases such as "car insurance" with a CPC (cost per click) in the range of $60 to $70. Yes, car insurance companies pay this amount every time someone clicks on their ad. All keywords and phrases won't be this competitive but this is just an example. Your Cost Per Click can be less than a dollar in some cases so don't be too alarmed.
To answer your last question, the purpose of PPC advertising is to get your ads in front of your audience and ultimately drive traffic to your website.
It is quite impossible to explain it all to you in a single post but hopefully I have provided some perspective.