Ammo boxes are exactly what they sound like: boxes of ammunition. When you see ammo in the wasteland, it's never sitting around in the form of individual bullets; you always pick up little boxes or magazines that look like these:
(sorry, Xbox 360 + digital camera + MS Paint; if there's a better way to get a screenshot, let me know)
The same graphics are used for equivalent ammo types (e.g. .38 Special / .357 Magnum), alternate versions of the same ammo, (i.e. hollow point and armor piercing variants) and bulk/surplus versions of the same ammo.
Some, but not all, ammo boxes contain bulk ammo. Look for the word "bulk." Here's what I got with my bulk energy cells:
Gun CND × 0.85 / DAM × 0.85. In other words, bulk energy cells do only 85% of the damage of regular energy cells, but also only degrade your weapon 85% as quickly.
To make things even more confusing, there's also surplus ammo. I'll let Chet from Goodsprings explain:
"You'll be cleaning and fixing your gun a lot more than usual when you use [surplus ammo]."
For example, the "effects" for surplus 5.56mm ammo compared to regular 5.56mm ammo are
Gun CND × 3.00 / DAM × 1.15 So you get 115% damage output, but your weapon degrades at 300% of the usual speed. You get what you pay for.
Note that I have not tested all bulk/surplus ammo types, and others may have different ratios for condition or damage or other effects.