In the United States today, lethal injection is almost the only form of execution that is used. All of the states which have the death penalty, as well as the federal government, use lethal injection as their primary method of executing condemned prisoners.
In the past, a variety of other methods of execution have been used. In relatively recent times, the electric chair, the gas chamber, hanging, and firing squads have all been used. Of these, the electric chair has been by far the most prevalent in recent times. As you can see by following this link, there have only been 17 executions by a method other than lethal injection or electrocution since 1976.
Lethal injection has become the most common form of execution because it is believed to be the most humane possible way to kill people. The death penalty has been under fire by many groups, some of whom try to portray it as cruel and therefore unconstitutional under the 8th Amendment. For this reason, all states have moved to lethal injection, though there are a few states that still allow prisoners to opt for other methods of execution.