As Tom alluded to, SWTOR does have shards on a per server basis. If you are in a different shard from someone else you won't see them, but you can still see their chat. If you try to group with them you will get switched to their instance so you can physically interact. This allows more people to be in common zones without having to fight over mobs/ quest items/ etc. it works really well and makes the game playable when the server is full without losing the 'massive multiplayer' feel.