There are total 3 match officials in a cricket match. 2 on field umpires and 1 third umpire.
On field umpires make almost all the decisions and only reviews are given by the third umpire, also known as the television umpire. Basically on field umpires are almost everything for a game.
There are 2 on field umpires , one stand behind the stumps on bowling end and other on the leg side of the field. The umpire standing behind the stumps is the main umpire. Although they interchange after every over.