What are the different types of military uniforms and occasions for wearing them?

Question

ALSO why were most of the soldiers I saw yesterday wearing their button up shirts and slacks instead of the usual ACU's or BDU's or whatever their called?

Answer 1

they were wearing class A's or B's

the military or the army in this case were 3 types of uniforms that i know of

class A= are the dress uniforms they are used for formal events

class B= are the working uniforms but the arn't really used as much

class C= are the feild uniforms like the ACU that is the most commonly used one  

Answer 2

As for U.S. Army uniforms, see AR 670-1

http://www.gasdf.com/regs/AR_670_1.pdf

Answer 3

In the Army like they said above, Class A is a Coat worn for formal events and stuff like that. Class B is Just the shirt without the coat. Also worn for events like those. Class C is the ACU's. Most comfortable IMO. Worn in battlefields.