Combination Reaction - when 2 substances react to form a single product.

A+B - AB

Decomposition Reaction - when a single reactant breaks to two or more products 

AB -> A + B

Single Replacement Reaction -  a single free element replaces or is substituted for one of the elements in a compound 

A + BC  ->  AC + B

Double Replacement Reaction - Involved in the exchange of ions by two compounds to form two different products

AB + CD -> AC + BD