Synthesis, decomposition, substitution and double displacement are four types of chemical reactions. A synthesis reaction is when two or more reactants combine to form one product.
When a reactant is broken into two or more products, a decomposition reaction has occurred. Water can be broken into hydrogen and oxygen molecules.
A substitution reaction occurs when a single element replaces an element in a compound to form new products. A solution of zinc and hydrochloric acid reacts to produce hydrogen and zinc chloride.
A double displacement reaction occurs when two compound reactants exchange an element with each other to form new compounds.