I read on the facebook TESO page FAQ page this :

How does multiplayer work?
Players can choose to join together in small adventure groups or large communities such as guilds. As part of a group, you can work together to solve quests, head into epic battles or fight for the throne of Tamriel.

Does this mean that the world will be more instanced, like GW1, where you only travel the world with those in your group/guild? Or does it mean that you are able to explore the world openly with others as in WoW/GW2.

It's more like... ESO?

You'll be able to travel your alliance freely on one character, others on other characters. There will be some instanced dungeons, but open dungeons as well.

It's open world (like GW2) but instanced because it's the megaserver. It will put you with those in your guild, or who you've grouped with before or even based on what you put in the survey.

So you're instanced, but the instance is persistent for you. IF that makes sense.

Think of the instances as being different shards except you can move more freely among them.

It's like having your own copy of the entire world and sharing it with people that have similar interests. It doesn't have loading screens every time you want to leave the city, it's persistent as far as I know.

It's more like WoW and GW2 except you can pick who to play with.