Deeep is a game about life. Or, more precisely, it's a game about eating your way up the food chain. You begin as a small fry, and then gradually work your way up, consuming anything smaller than you while avoiding anything bigger than you. Many of the different creatures you can play as have different bonuses when played a certain way in a certain region of the map.
The controls are simple. Move with your mouse. Eat with your mouth. Boost with the left mouse button, and follow the directions on screen to take advantage of any special bonuses you have.
Eat enough food, and you can evolve to the next fish up the chain. Survive for long enough, and you'll be a big fish in a small pond.
Play to your creature type. If you're a frog, don't try going deep into the ocean and living with the angler fish. They'll just think you're weird as they eat you.