Nervous System

Function:
Controls everything you do from picking a coffee cup up to typing on a computer.

Organs: 

Brain (the control center), Spinal cord (the highway for information to the brain and from the brain) and Nerves (connect your body to the brain).

Interactions with other systems:

The nervous system usually works with all other systems to give them orders like when to move your muscles or when you smell your grandma's apple pie through the kitchen door. But it doesn't work with some because they act on their own. But it does work closely with most systems like the muscular system.

Analogy:

The nervous system is like a CPU because it carries out instructions to systems like the muscular system for when to move and stop and when to move a arm and where to stop it and use the fingers to pick up a coffee cup.

Structure and Function:
Without your brain you would be mindless, a zombie without any control center to tell its body what to do.  The brain is the control center, without it your body wouldn't to be able to something as simple as picking up a coffee cup and drink it. The spinal cord assists the brain in sending messages down to the body where it sent all over to tell the body part what to do. The size is unique to fit in the skull of your head so it can be surrounded by a thick wall of bone to prevent as much damage as it can.

Sources:
Biology