A keyword is anything that a person is likely to type in a search engine to locate information or website's. A keyword phrase is simply 2 or more keywords entered for a search.
Targeting a single keyword is not recommended, with billions of words indexed on the web, generally one keyword will bring back too broad, or irrelevant information. Statistically, most people search on two, three or four word phrases to avoid getting unrelated matches. Try thinking like your potential customer, what phrases would they use, this is the target group of phrases you need to come up with?
Put yourself in the position of the visitor you want to attract. List what keywords or phrases they are likely to search on. This requires you to separate yourself from your product, service or website and think like a person who knows nothing about your site. It is good to ask friends or coworkers what they might type in to find your site, or even have them go to a search engine and tell them to try and find information on the topic your site happens to cover. Watch what words and phrases they use.