SEO Best practice #1 - Keyword usage and Targeting

Keywords are fundamental to the search process - they are the building blocks of language and of search. In fact, the entire science of information retrieval (including web-based search engines like Google) is based on keywords. As the engines crawl and index the contents of pages around the web, they keep track of those pages inkeyword-based indices. Thus, rather than storing 25 billion web pages all in one database (which would get pretty big), the engines have millions and millions of smaller databases, each centered on a
particular keyword term or phrase. This makes it much faster for the engines to retrieve the data they need in a mere fraction of a second. 
Obviously, if you want your page to have a chance of being listed in the search results for "dog", it's extremely wise to make sure the word "dog" is part of the indexable content of your document.