ACU stands for Army Combat Uniform. The US army transitioned to the ACU in 2005. Beforehand, we used the Battle Dress Uniform (BDU).

Includes

The ACU has a few different articles of clothing in the set, such as:

• T-Shirt- 100% cotton and moisture wicking
• Jacket- To end above the cargo pant pocket, but below the side pant pocket. Sleeves to be worn down. It may be substituted by the flame-resistant Army Combat Shirt, to be worn directly under the Outer Tactical Vest.
• Trousers- To be worn with a nylon belt and not to be worn below the third eyelet of the boots. Includes plenty of pockets and is Identical to the Army Combat Pants except for it's flame-resistance.
• Footwear- Tan, rough-out combat boots and moisture-wicking socks.
• Headgear- A patrol cap with a thick bill and inside pocket

All of these utilize Universal Camouflage Pattern technology, and NIR Signature Management technology (hides infrared silhouette).

BDUs (Battle Dress Uniforms) were the combat uniform of the USA from 1981 until 2005.  They call it the BDU because it is meant for battles, and not for public apearances.  There are many types of BDU. 

Desert Combat Uniform (DCU)

The DCU was created to replace it's predecessor, the Desert BDU of the 1990's.  

Because patches of color are about ten times bigger in the desert than in the forest, the DCU has a three color camouflage instead of the previous six color camouflage.

Airman Battle Uniform (ABU)

It is almost identical to the color of the ACU, but also includes slate blue.  Unlike the ACU, the sleeves are authorized to be rolled up.

 By 2010 it will be the main uniform worn by the US airforce.