OK... here's a quick example... In a heraldic blazon, if you state that the arms are: Per pale gules and argent, you are indicating that the field is being divided into two parts, using a vertical line, and the leftmost portion od gules (red) and teh rightmost is argent (white). This is an example of a field division. For the purposes of heraldry, this is considered to be the same layer of the Arms.

If, on the other hand, your blazon is: Gules, a pale argent, you have a gules (red) field, and a pale argent (a vertical stripe usually 1/3 of the width of the arms in white) centered on the field. This is adding a second layer (the pale) to the gules field.

Th fun part is when you place charges (layers) over to of each other -- since you will need to maintain good visual contrast between the layers.

This is usually done by making sure that you place colour on metal or metal on colour. placing metal on metal or colour on colour will not give you good contrast and will mess up identifiability at any sort of distance.

There are two metals used in Heraldry -- argent (white) and Or (yellow)
There are five colours used -- gules (red), vert (green), azure (blue), purpure (purple) and sable (black).

Thus ends the first class of Heraldry 101.