Single-replacement

A single-replacement reaction is a type of oxidation-reduction chemical reaction when an element or ion moves out of one compound and into another. (One element is replaced by another in a compound.) This is usually written as
A + BX → AX + B
This will occur if A is more reactive than B. You can refer to the reactivity series to be sure of this.

A and B must be either:

• Different metals (hydrogen's behavior as a cation renders it as a metal here), in which case X represents an anion; or
  
• Halogens, in which case X represents a cation.

A single-replacement reaction example:

Fe + CuSO4 = Cu + FeSO4

http://en.wikipedia.org/wiki/Single_displacement_reaction