Portal. Limbo. Shadow of the Colossus. Braid. Passage. Journey shares one thing in common with these unique games: I want to share it with other people. I want others to experience this game, whether they're gamers or not. I'll share Journey with my parents, who don't even pretend to understand my hobby. Journey's art, music, and gameplay blend flawlessly into one of the most unique and satisfying cooperative games that I have ever experienced.
You begin Journey alone...
To explicate Journey would dull the experience. I could cover the simple game mechanics and explain the implementation of co-op, but that's something better left discovered for yourself.
... but you'll soon be joined by another.
I will say that you will be compelled to play Journey cooperatively. You will figure out how to communicate with your random co-op partner, and you will see benefits from interacting with each other.
How you treat one another will influence both of your experiences.
I have played through the game twice, and both times I had a different cooperative encounters. I plan on enjoying several more playthroughs, and not for something as trifling as Trophies, though they are awarded for cooperative play.