Keyword Ads are online advertisements that are determined by the particular keywords you use as you search for something online.

Google uses keyword ads quite a lot. These ads often show up on other webpages in a small sidebar box labeled "Ads by Google."

The benefit of keyword ads over other forms of advertising (like banner ads or pop-up ads) is that keyword ads are actually related to the content of the page the user is viewing, which makes them more likely to "work"--people might actually buy the thing being advertised!

Google uses cookies to help determine what sort of ads you will see. Based on your past searches and things you have clicked on, the information stored in these cookies helps Google target advertising that is closely related to what you are more likely to actually buy. The downside of this is that they are (possibly) collecting your personal information that you might prefer to keep private in the process--which might make your information less safe.