Definition: A marketing system on the Web in which the advertiser pays when the user clicks on its advertisement and goes to its site. This is a more interactive, results-oriented method compared to paying for just the placement of a banner ad on a Web page regardless if anyone clicks on it. Pay Per Click is paying a web site vendor for a visitor's click on an ad that sends them to your web site. Most of the vendors, largest being Google and Yahoo, do not charge you until the visitor actually clicks to your site.
Content Matching is when your ads are shown on sites that have similar content or related content. In real estate, this could mean that your ad would show up on a travel site on pages related to your area. Depending on how your ad is worded and the content of the page to which it brings the visitor, the click vendor's system selects web pages on which to display your ad. The display of the ad creates an "impression". The visitor sees it, but may not take action by clicking on it.
Keyword and Phrase Matching is the purchase of positioning in the search engines at the top for certain keywords and phrases that you believe will bring targeted traffic to your site. In our business, the big ones are "YourTown real estate", Yourtown, State real estate" and similar combinations. Advertisers are actually bidding against each other for top placement on searches for those keywords. There are complex ranking formulas that may not rely solely on highest bidder gets highest placement, but that's the normal method. Again, no payment is made until the visitor clicks to your site.