Infantry is the main part of any fighting force, or at least it always has been. This may be changing as warfare comes to be more dominated by "smart" weapons. However, in the past, infantry has always been the part of the armed forces that must take ground and control territory.
There have been many times when infantry has been thought to be losing importance. There was, for example, the Persian Gulf War of 1991 when it was hoped that air power alone would defeat Iraq. There were those who thought the same could be done in WWII.
In each case, they have been wrong. Air power and artillery and such are necessary, but infantry has to actually go in and take ground in order for most wars to be won.
Infantrymen are the soldiers on the front line of a battle. These soldiers are sometimes known as foot soldiers as they are trained to engage in battle on foot, face to face. Typically these men suffered the highest causality rate.