Infantry units are the most basic unit type available. They are low cost, but are mostly unable to fight tanks or helicopters. The main reason to use most infantry is for cheap firepower or defence, though some infantry is useful in all situations.
The four primary types of infantry are Light Infantry, Heavy Infantry, Commandos, and Engineers. Light Infantry, Heavy Infantry, and Commandos are all combat-oriented infantry, while Engineers deploy traps to disrupt enemy forces.
The biggest advantage of infantry over vehicles is that they recieve the largest benefit (and penalty) from cover with a cheap price, allowing a fairly effective army to be deployed on defence, where positions with good cover can be maintained.
When using infantry, make sure they do not attack from or end their turn in clear or road terrain, as the penalty for these can cripple the odds of the infantry in any battle, even lowering Armor to below 0, where nearly any attack will eliminate them without effort.