What is the keel on a ship?

The keel of a ship is like its backbone. It runs longitudinally along the center of the bottom of the hull from stem to stern. Traditionally, it constituted the principal part to which the ribs were attached on each side and to which the stem and stern-post were also attached.
In sailing ships made of wood, it was the spine of the vessel, running from front to back (fore to aft) for the whole length; the 'ribs' of the craft were attached to it and it formed the basic structure for the building of the ship. Most modern ships do not have a keel as they have rounded, smoothed or flat bottoms, but some have it as a seam between the parts forming each side of the ship.
hmm i know what it is but trying to put in less than 500 Words,is hard.

it's the spine of the ship or breast bone depending on how you look at it.It's the part of the hull(shell that holds water out) that runs all the way from the front to the rear and is sort of a blade that cut's the water.A flat bottom boat would not have one.

Now I can give a much better explanation( as this is way over simplified) but it would confuse most people not familiar with terms like starboard Bow and this is all you need to know if all you want to know what it's called and what your looking at on a ship, so if you want a better explanation of what it dose and how it works try here http://en.wikipedia.org/wiki/Keel

Hope this helps.
A keel is a large beam along the underside of a ship's hull.