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Keywords are the words that people type into search engines like Google and Yahoo when they want to find information. For instance people looking for house plant information might type ‘house plants’, ‘houseplants’, ‘house plant care’, etc.

When you write your blog entry you want to use those same keywords in your blog entry. And use them in the post title.  This will give you a higher placement for those terms and make it easier for customers to find you.

There are several free sites that will help you find and choose keywords:
Overture excellent site when up.
SEO Book Keyword Suggestion Tool
Free Keyword suggestion tool