Google Adwords is a service that displays advertisements on result pages and partner/affiliate sites when people enter related keyword search terms.
The service has been designed to make it easy to publish advertisements. With a very low price entry point, you pay only when someone clicks your advertisements and is taken to the advertised website.
Ads can also be configured to be shown at specific times, geographical locations and on certain websites. This level of flexibility has made Google Adwords one of the most popular advertisement networks while also being highly contextual.
Even though the service is called AdWords, users are also given the option of using images and videos in the adverts.