Key phrase
What is a key Phrase?

key Phrase

Some of these key phrases are more valuable than others (from the point of view of the SEO) because people do their search using some particular key phrases more than others. The popularity of key phrases change as time passes making it necessary for the search engine optimization professional to make the necessary changes for the ranking and competition level of the key phrase in order to get as many qualified visitors* as possible. There are different ways of finding out which keywords are the most popular for a particular subject.

These websites can help you to find the right keyword/key phrases for your site:
 

Overture
http://inventory.uk.overture.com/d/searchinventory/suggestion/

Called Keyword Assistant, it is a tool used by search engine optimization professionals for finding related searches that include your term and how many times that term was searched on last month.
 

Word Tracker
http://www.wordtracker.com/

Word tracker compiles a database of terms that people search for. Enter some keywords, and they will tell you how often people search for them, and also tell you how many competing sites use those keywords. Wordtracker helps to find all keyword combinations that bear any relation to any business, service or product - many of which you might never have considered.
 

Google AdWords
https://adwords.google.com/select/main?cmd=KeywordSandbox

This tool can provide ideas for new keywords that can help you improve your rankings. Although the AdWords research toll helps to find all keyword combinations that bear any relation to any business, service or product it does not provide the number of times a term have been searched. It was primarily created to help people find the best terms in their AdWords campaign.
 

Good Keywords
http://www.goodkeywords.com/

Good Keywords is a free Windows software for finding the perfect set of keywords for web pages. Knowing the right keywords to target is very important because your site should provide what people are searching for.

(*) Qualified visitors - potential customers that come to your website. In one way or another these visitors are interested in what you are offering. Using tactics to attract visitors that are not interested in your products or services can  irritate the visitor, waste server resources and can hurt your rankings.