Infantry is the most basic unit in game. They're most effective fighting against other infantry and capturing cities, but vulnerable to tanks and artillery. They are the most spammable troops in the game, requiring no extra resources to train and can be trained in multiple cities at once with no restrictions. Hills and mountains can reduce their mobility, hindering their progress.
Infantry can receive a defensive boost when they stop in a city and wait two weeks, it's called Entrenchment. Since the Technology Update, Infantry can be upgraded a lot and also they can be effective against tanks too.
Infantry is the cheapest ground unit. They cost one million dollars with 20 thousand dollars upkeep.