Let me imagine that you have no idea what Contra is like and need a description. You, or you and a friend together, take the role of a commando sent to a tropical jungle to eradicate an alien threat. For most of the game, you run to the right across platforms, shooting bullets across the screen to destroy any enemies in your way.
In two base stages, the view changes to behind your commando's back. The stages look 3D, but you are still constrained to move left and right (until you blow out the room's back wall... but as you move forward, you will not be attacked).