Keywords can be classified into three categories :

-Single word Keyword
-Multiple word Keyword (Long Tails Keywords)
-Keywords based on Theme

Keyword(s) are basic raw material used in Search Engine Optimization (SEO). Keyword selection or Keyword Research as it is called technically, where we use special tools to find out a list of Keywords (search terms) searched by targeted audience, recently. Keyword can be single word, two or three words, multiple words and theme based.

Lets detail each of these one by one . Understanding these categories of keywords would also help one to decide as to their targeting on specific pages.