Conditional Probability and the Probability of "AND" & "OR"
In the following diagram:
The circle on the left represents the part of the sample space in event A
The circle on the right represents the part of the sample space in event B.
This means the overlap of the two circles represents the part of the sample space in BOTH event A and event B (a.k.a AnB!)
This means that the total space covered by the two circles represents the part of the sample space in AT LEAST ONE OF event A and event B (a.k.a. AuB!)