Question.- Noob

So I was curious. When playing ESO, are you playing with players across all platforms (PS4, PC, MAC, Xbox)? Or is each platform given their own ESO world so to speak?

For example, I have PS4 and just purchased ESO Gold Edition yesterday. My sister in Idaho has ESO but for Xbox One. Will we be able to play together? Or because we are using different platforms will we only be playing with members from our own platforms?