Forces and their reactions

A body of mass $m$ is placed on a table. The earth is pulling the body with a force $mg$ . Taking this force to be the action what is the reaction? Or more generally, Is it true that the reaction of a gravitational force is always gravitational, of an electromagnetic force is always electromagnetic and so on? If I hang a block of some mass $M$ with an ideal string, is the tension in the string the reaction of the force applied by the Earth? In other words, are the tension and weight an action-reaction pair? My teacher told me they are, but I really doubt that. Also, could you suggest me some source to study about forces and Newton's laws of motion from the very basic, because I'm bit shaky in my concepts. Any video link or book would do. Thank you $\smile$