When a call queues it generally waits for the next available agent.  However, a call can queue physically or logically dependent on the number of ACD agent groups.

To take a view outside the call centre lets look at supermarket queuing.  When waiting in a check-out lane, you are in a physical queue and wait your turn.  However, someone could join a 2nd check-out lane after you, but still get served before you.  You cannot intercept another queue and say it is your turn - if you could that would be logical queuing.  So back to the call centre;

Physical queue - occurs where a call waits for an agent group and can only be answered by this group.  There could be any number of physical queues which could be set by skill or product.

Logical queue - occurs where a call queues against it's primary group but also overflows and queues against other groups at the same time.