Answer 1

A queue usually means that the order of adding and removing with the container follows FIFO (first in first out) similar to people lined up at a grocery store.

A queue normally selects the oldest inserted value for removal.

A priority queue removes elements based on some other rule. It'll select based on "priority" and you define what priority means. A priority can be defined by a comparison function that says what of any 2 arbitrary elements has higher "priority".

What happens if the highest priority is always the oldest time inserting an element? Well, that priority queue is also a very conventional queue.

FIFO and non-FIFO order is the critical difference between a conventional queue and a priority queue. Beyond that, usually the data-structures are different for more efficiency. For example, a linked list is great for a conventional queue but a heap is great for a priority queue.