What is the official called who oversees the game of cricket from behind the wickets?

The person who you are thinking of would be the umpire standing at the non strikers end. They stand about one meter or so inline with the stumps as to give them the best view of the batsmen's stance in relation to the stumps and the keeper. This gives them the best possible chance of making the correct decision when it comes to an lbw or caught behind appeal.

There are many other officials who oversee the game, with the other on field umpire being the square leg umpire (who, unsurprisingly, stands at square leg). The main umpire and the square leg umpire take turns in the relative positions.

There are also the people who hold the official of positions of third and fourth umpire, but they are not on the field of play.