Tokyo Jungle is kind of a "what you make of it" sort-of game. There are two primary modes, Survival and Story. In Survival Mode, you pick an animal and essentially survive as long as you can. It's an incredibly arcade-like experience. You earn points by either completing randomly generated objectives or surviving for a set amount of time. These points can be used to purchase accessories or other animals that you unlock. It also serves as a high score for a leaderboard.
From there, you can collect Archive items in Survival Mode which unlock Story missions that play out almost like a tutorial for Survival Mode. There's cut scenes and dialogue that tell some sort of story about a Pomeranian and his journey through the "jungle". There is an "end" to the Story mode, but you need to play both modes, almost back and forth (from what I can tell) to see the "true" end. However, as most people have said, Survival mode is basically endless. You play as long as you survive. I'm not sure if you can live forever if you play well enough, but it seems like a major goal is to survive at least 100 years (which is probably a 2+ hour session, maybe).
