what is the prime difference between displacement and double displacement  reaction?

We know that displacement reactions are those reactions in which an element replaces another element from its compound. Displacement reactions are basically of two types 

1) Single displacement reactions

2) Double displacement reaction

Single displacement reactions are those in which an uncombined single element simply replaces the other in a compound. A general chemical equation for reaction is   

AB + X → AX + B

For example

A Double Displacement Reaction on the other hand is a reaction involving two molecules in which parts of two compounds are exchanged to give two new compounds. The general equation used to represent double displacement reactions can be written as:

     AB + CD → AD + BC

For example

Thus, single displacement reaction is the reaction between an element and a compound, while a double displacement reaction is the reaction between two compounds. Also, a single displacement reaction leads to the formation of a new compound and deposition of a new element, while a double decomposition reaction leads to the formation of two new compounds.