Too many to mention to be honest. If there is a certain something you wish to know just pop it in the search and I would be shocked if there was not a thread about it. Also just look here in the newbie section so so many questions have been answered.
Dolker right:
Use this link https://adwords.google.com/select/KeywordToolExternal to find out keyword popularity. While Google pay more for single keyword such as "loan" less for two combine keyword such as "UK loan" etc.