What is the difference between a forward, aft, high and low center of gravity?
I'm not sure I'm understanding what you're trying to get at, but let me try to answer the sentence:


  most importantly when is the CG is AFT or Forward?, and when is the CG Low or High?


Whether the CG is aft or forward refers to its position along the longitudinal axis of the airplane, that axis that runs from the aircraft nose to the tail. Usually if people refer to just "the CG" they're typically referring to the longitudinal position of the CG as that is generally the most critical. Boeing refers to the longitudinal axis measurement as the BA, balance arm, although the longitudinal CG is usually spoken of in terms of the percent of the mean aerodynamic chord.

Let's say you have a freighter with a single 10,000 lb pallet in it. If that pallet is in the forward most position capable of handling it, the longitudinal CG would be considerably more forward than if the pallet were in the aft most position. Very possibly in either instance the respective forward and aft CG limits would be exceeded. If a loadmaster said only, "I've got a forward CG," he might mean that the load is forward of the forward limit, or he might simply mean that the load is within the limit but more forward than he likes.

Now let's say that we split the 10,000 lb pallet into two 5,000 lb pallets, and we put one of them in that most forward position on the main deck and the other in the same longitudinal position on the lower deck. That wouldn't change the longitudinal CG because both pallets are at the same longitudinal position. However, it would change the vertical CG as measured along the vertical axis. Loadmasters on 747s don't have to worry about the vertical CG because there are no defined limits for it in 747 weight and balance manuals. Loadmasters on 767s, however, do as there are defined limits. If a 767 loadmaster says he has a high CG, he can mean that the maindeck load is too great given how much is in the lower deck. He also might mean that the vertical CG of the maindeck load itself is greater than 42" above the maindeck floor. Boeing refers to the vertical axis measurement as the WL, water line.

Though you didn't ask about it, there is a third axis, the lateral axis, extending from wing tip to wing tip so to speak. Boeing refers to the position along this axis as the BBL, body buttock line, and there are typically limits as to how far from the aircraft centerline the lateral CG can be.