Sometimes people use it to describe old school fast paced arena shooters. And sometimes they mean modern "realistic" games like CoD - where in a gunfight winner is decided by who saw who first and twitched his aim, since everybody dies in a few hits.
Just use terms like "arena shooter" and "modern military shooter" to avoid confusion.
Anyway, returning to the topic, Planetside 2 gameplay is pretty much like Battlefield: you have lots of people fighting each other both on foot and engaging in vehicle combat. The game has control points, squads and classes. Only it's an MMO shooter with all the atributes: it has persistent world and it's (obviously) massive.