When writing an ads, it is hard to overstuff your ad with keywords. Overstuffing would be an exterme amount of keywords that don't actually make much sense, but just have keywords.
If you write a normal sentence, you might include one keyword. For example:
Keyword: Blue Widgets
Example.com's Widgets
Find the cheapest widgets
shipped same day.
www.example.com
This ad is a generic ad that could be used for any number of widget sales ads. This ad doesn't include one of the main keywords, blue, and is boring. It is boring because there is nothing that draws my attention from the main search results to this ad.
Cheap Blue Widgets
Find the lest expensive
blue widgets here.
www.example.com
This is a good ad, it has keywords in the title and in the body of the ad. The keywords seeem natural and the ad can be read aloud without any trouble.
When writing an ad, you have to use keywords. Since you need to use keywords, you need to have different ad groups setup so that you have at least one ad running for each major keyword.
If you sell widgets, you should have separate ad groups for the following keywords:
blue widgets
red widgets
cheap widgets
used widgets
etc.
If you just had one ad that incorporated each of those keywords your CTR would be low because of that lack of exact keywords.
Keywords are the most important facet of any great ad, don't be afraid to use them and test the results!
