A player ally is an NPC or a bot that will help the player, by providing covering fire if the ally has its own weapon. Allies can also be used to open special locked doors, with keypads If scripted so.
The difference between an ally and a Player Companion is that a companion is normally a long term helper, Examples include Alyx and Barney. Allies accompany you temporarily. Examples include: Citizens, Refugees, and Rebels.
The only main way that this branch of AI specificaly differs from others, is that it automatically Likes the player, though this can be changed with the aforementioned ai_relationship.