The Law of Superposition
This law says that in any undisturbed piece of rock, the oldest layer is at the bottom, and the youngest layer is on top.
The Principle of Original Horizontality
This states that any rock layer is deposited horizontally. So, layers are always originally horizontal.
The Cross Cutting Law
This states that anything that cuts across a rock body is younger thank the body it cuts across. The reason for that is that the rock body must have been there first in order for it to be cut into.
Principle of Inclusions
This states that if a rock, or inclusion, is entirely inside another rock layer, then it's older than the layer around it. This can be explained by understanding that the outer layer formed around the intrusion, which was already there, making it older.