Infantry or foot units represent the most basic units you can train, and encompass all non-mounted Citizen Soldiers and non-mounted champion units. They're usually the most inexpensive and cost effective to mass in numbers with an exception to heros and champions, while having less housing requirements. They're one of the most flexible units available capable of garrisoning into almost anything that is garrisonable, from defense towers to siege rams.
Some units have a specific bonus against all infantry, or a specific kind of infantry. Infantry Slinger has a general bonus against all infantry, more so for ranged infantry such as Infantry Archers and less so for melee infantry Infantry Spearman.