Electrical is in charge of wiring the robot. Motor controllers, pneumatic systems, and sensors are all part of their purview. They design the nervous system of the robot, with everyone relying on them to make sure that signals get from one point to another.
Marketing runs everything on the team that isn't directly focused on building robots. They organize demos, track budget, and of course write our chairman's essay. Without them, the team would be unable to work in the community or help spread FIRST.
Mechanical, of course, builds the robot. They design, CAD, and machine parts, before putting it all together into a single final product. Often the largest sub-team, they divide further into 'upper' and 'lower' mechanical, with lower working on the drive base of the robot and upper working on whatever specific system the game requires that year.
Programming writes the code that runs the robot. Without it, the robot would be effectively an extremely expensive sandbag. On top of just the raw code, programming creates diagrams and documents keeping track of the robots function, and aims to make the drivers job as easy as possible, so that they can focus wholly on the game, not on a finicky robot.