An AdWords ad is an ad which is displayed on Google result pages or Google partner websites (AdSense Network).
On Google’s SERP, ads appear above, below (more recently) and on the right side of organic results. An AdWords ad is often referred under the term of sponsored links.
The standard ad created by the advertiser is made of visible elements (headline, description lines, display URL) and an invisible element (destination URL).
Ad extensions offer several other formats for Google Ads.
For standard ad, the headline can contain 25 characters maximum, spaces included, the two description lines 35 characters each and the display URL 35.
The ad creation interface and matching ads:
An ad is linked to a selection of keywords at ad group level. It is possible to create several ads for the same group which will appear in a random rotation on requests matching the keywords list.
Ad writing optimization plays an important role in sponsored link campaigns performances.
For more details on AdWords ads operating principles, see Google AdWords.