One primary god, that determines the scales (and any dominion effects) and blessings. X number of 'disciples' who can choose magic paths, which are basically prophets (with a pretender chassis) without the holy levels that can be called back like a normal god and claim thrones. You all push the same dominion. You can relinquish provinces to allies and sitesearch allied provinces, though you can't build on them (you can, however, have them relinquish the province to you, you build the building, and then hand the province back). Most (beneficial/dominion effect) global enchantments effect all allies instead of just the nation casting it. That's... really about it insofar as mechanical differences go, so far as I can recall.
Beyond the mechanical differences, it's like having an enforced permanent alliance that can actually share a victory. Adds a lot to the playing experience, in my experience, having people that actually are guaranteed to have your back, that you can bounce ideas off of and coordinate with without trouble, etc., etc.
Personally, I'd say it's a fairly substantial improvement over the base mode of play. Adds a lot to the game in terms of interactions and whatnot, and generally makes it more fun/interesting/engaging/etc. It's good stuff, and I'd totally recommend it.