While most items will typically use an inventory slot all to themself, there are some items that will "stack" on the same slot. Typically these items represent something that it is expected you will have a large number of, or perhaps are small in nature. The number of items that are stacked on a single slot is located in the lower-right corner.
To separate a stack, hold the shift key as you click on the stacked item and a small dialog window will appear asking how many of the item in question you wish to move.
Most items will stack to 5, 10, or 20. Some particular items that are often represented in large quantities will stack to 50, 100, or 200. Following patch 3.1, ammunition stacks in quantities up to 1000.