To paraphrase PhillyBaby, A nose heavy airplane doesn't fly well, A tail heavy plane doesn't fly long.

A plane that is nose heavy will tend to fly "Sluggishly" and will have a tendency to not slow down well on landings, while a tailheavy plane will be extremely manuverable.

BUT!

There is a fine balance here. Imagine the CG as an invisible line that goes through the airplane. The MFG might recommend let's say 1/4" on either side of that line as a recommended balance point. If the CG is in front of that range, it will still fly, (although sluggishly). But if the CG is BEHIND that line, you will most likely be bringing your plane home in a paper bag.

Think of an arrow (It has been said that the guy who invented the arrow was the worlds first aeronautical engineer); straight shaft of uniform weight, heavy, pointed weight on one end, Large, lightweight fins on the other. As it travels through the air, the point penetrates the air, while the feathers are held back by the airlflow.

Now imagine taking the point off, and making the feathers out of lead. If you shoot this arrow, as soon as it leaves the bow, the tail will whip around to the front because it's heavier.

This is exactly what a tail heavy airplane wants to do.

If you are not sure of where a plane should balance, Start by balancing it just slightly forward of the thickest part of the wing. Then fly it and see how it responds to controls. If you're capable of doing aerobatics, take it up and see if it will do a spin. If it won't, (or it's difficult to enter a spin) it's nose heavy. Keep moving the CG back until the plane flies the way you want it to. But keep in mind that as you move the CG back, the plane will get more responsive, and that there IS a critical point that you cannot go past.