PPC stands for pay-per-click, and when most people refer to PPC they're talking about Google Adwords, the largest paid traffic generator on the web. PPC advertising means that you run your ad for free, but whenever someone clicks on the ad you have to pay. PPC ads can run as low as $.05 a click and higher than $20 a click, and for high volume terms you can very easily spend thousands of dollars a day. PPC ads are bid based, meaning that you enter a bid and the advertiser with the highest combination of bid price x click through rate (in addition to a Google assigned "quality score") gets the best position among the displayed ads. The higher you bid, the higher your ad will show up.
If you're selling a product PPC may be a great way to generate sales or increase your profits. But if you're monetizing your site via affiliate marketing or Adsense it rarely works out due to the cost of each visitor in comparison to the revenue you make per visitor.
Google is not the only company with a PPC network. Facebook for example has a similar program that allows for even better customer targeting, but in many cases Facebook ads are less likely to convert due to the reasons most people use Facebook vs. Google search. You can also try PPC programs with Microsoft, other small search engines, and a variety of social networking/dating sites. We'll cover the basics of PPC below.
Generally you'll find that the big, high traffic head terms don't convert to sales nearly as well as specific long tail terms. For example, if you run an ad for "widgets" you may get a lot more visitors than for "brand x red widgets", but far less sales. Because each click will cost you, you're better off bidding on terms that are more likely to lead to a conversion or sale. Aside from choosing keywords related to your industry, you should also consider bidding on your competitor's domains, vender names, product names, and with modifiers in front of your keywords that are more likely to lead to a sale, such as "buy brand x widget".
There are three different match types in Google Adwords, meaning three different ways you can choose to bid for keyword phrases. Broad match will have ads show up anytime a person searches for a term that includes the words in your phrase, even if they're out of order. This is almost always a terrible option, as you'll be spending money even on people who are definitely not going to convert to a sale. Phrase match is the second option, and while it's better than broad match you'll also likely end up spending money on people who will make you nothing. Exact match will allow you to only bid on searches that exactly match your targeted terms. You should begin with exact match bids and only move to phrase or broad match if you're doing extremely well with exact and want to tread carefully as a test.
With Adwords you have the option to display your ads on both Google's search network (Google and other search partners who use Google search) and Google's Content Network. The content network includes any site that uses Adsense. If you do decide to display ads on the content network, which can often be as profitable as the search network, be sure to manage your bids separately. You can generally bid less on the content network than on the search network and get the same positions.
Facebook's PPC program may be worth testing depending on the product you're selling. The great thing about running Facebook ads is the incredible level of targeting they allow. You can choose whether you want your ad to display for males or females, age groups, based on what people like, etc. If you know exactly the kind of person who is going to be interested in your site or product, Facebook advertising can be a terrific source of customers who are likely to convert to a sale.