PriorityQueue only guarantees that the first element is the smallest.
A binary heap only guarantees in each sub-heab (sub-tree) the root is the smallest element.
The heap is actually a complete-tree (or an array representation of it). Whenever you insert an element that violates the condition (is smaller then the root), the old root is sifted down. this is done recursively over the heap.
This partial ordering allows fast and relatively cache-efficient (with array representation) data structure that can be used if you only need the min element at each point at time.
