The stuff you get from the vendors before lvl 50 is the same as PvE gear. The level 50 and up PvP armor has an extra stat called expertise on it. This stat is a direct increase to damage and reduction to dmg received during PvP.
The gear still works fine in PvE, but the expertise will do nothing, And because some of the item's stat budget is used for expertise, the other stats will be lower than a pure PvE piece of the same item level. (typically)