There are three types of Keywords:
1.) Generic Keywords
2.) Primary Keywords
3.) Long Tail Keywords
Generic Keywords includes brand name of product or website. Primary Keywords are through small one, two or three words which you want to search on search engine for your results. Long Tail Keywords are Combination of primary keywords. Mostly question vices long tail keywords prefer by optimizer. You need to find target audience for Long Tail keywords and you should have power of mind mapping to find exact keywords. Use Search Engine Manual research to find best long tail keywords.