There is 6 different units listed as Infintry. They are Marine, Grenadier, Chaingunner, Boomer, Marksman and Rain Maker. All except for the Rain Maker can be unlocked by having the corrct amount of resource to do so. Click one the names in yellow to go to the page for that unit. In order to unlock the Rain Maker you must complete the single player AI "Rain of Fire" and have the correct amount of resource.