What is the differance between light and heavy infantry

[Felix]

In earlier periods, the legions explicitly included light infantry - the velites, and in the "Servian" style army, the lowest classes of citizen soldiers. These types did not have body armour, didn't use the big scutum, and their primary armament were missiles - javelins, slings. In the Empire, light infantry were often auxiliaries. My understanding is that light infantry of Caesar's time were probably not part of the legion, but likely allies/auxiliaries; missile troops without armour (maybe without helmets), small shields if any, etc.