One of the basic components of a Google PPC campaign is the ad group.

Within the ad group, you write your ads and place your keywords — learn more about ads and keywords.

Google seems to prefer ad groups with commonly themed keywords. For example, Google would probably prefer this group of keywords:
 

orthopedic shoes
orthopedic shoes online
buy orthopedic shoes
orthopedic shoe store
orthopedic shoes website
 

to this group:
 

shoes
orthopedic shoes
running shoes
dress shoes
patent leather shoes
sneakers
 

Generally, an ad group should have two ads, and the ads should be different enough so you can do some comparison testing, known as A/B testing, and perhaps learn something about what your customers are responding to.

Here’s how to write a PPC ad.