heap - stack - pile
A heap of things is usually untidy, and often has the shape of a hill or mound.
The building collapsed into a heap of rubble.
A stack is usually tidy, and often consists of flat objects placed directly on top of each other.
...a neat stack of dishes.
Eric came out of his room with a small stack of CDs in his hands.
A pile of things can be tidy or untidy.
...a neat pile of clothes.
He reached over to a pile of newspapers and magazines