Your domain name is your website. Google examines every page on a website. Googles robots read every page, and every word on those pages. Each word found is counted toward the entire content of your domain and website. Therefore you need to target the content of every page toward your market or niche.

The common mistake is to have a single domain with many unrelated pages. For example, suppose a website had pages about lipstick, car tires and televisions. These are three unrelated products. The keywords are unrelated, and the content for these pages would be unrelated. When Google spiders the website, it will see various content on these unrelated pages. It will see unrelated keywords. Google will then decide your website is broad, generic website and label your domain and website as not an "expert" on any one keyword.

This is the problem. When a website gets labelled as a broad, generic website it becomes very hard to get a good quality score in AdWords. Since the domain will be thought of a rambling domain, it will difficult to rank for an specific keyword.

This is the exact opposite of what you want. You want to be considered an "expert" by Google for your market. This will help AdWords quality scores and search engine rankings. All of your pages, and therefore all of you content should be related to your market.

For example, if you wanted to have a good quality score or ranking for lipstick, your market would be beauty products. You should have a domain and website targeted this market, and only this market. You can have a page for lipstick, another for eye shadow, and another for blush. All of these products and keywords are beauty products.

Google will recognize the related keywords, and your content across all your pages will increase your quality score and ranking for beauty products.