Most of the search phrases online are three keywords in length, two keywords is the next popular number. You can select to do broad, exact and phrase. You do this either at your PPC network like Google Adwords, Yahoo Search marketing, etc., or you can do this manually by using these symbols to surround your keyword phrases: [] for exact, "" for phrase, and nothing for broad match. It's a good Idea to have all of the options. For example, say you have the keyword phrase Chicago Realestate. In your keyword meta data and elsewhere you enter the keyword data you would have chicago realestate, "chicago realestate", and [chicago realestate].
"chicago realestate" will bring up that phrase, but not the exact phrase. It would also bring up cheap chicago realestate or chicago affordable realestate. [Chicago realestate] will only pull up your site if someone searches exactly for chicago realestate.