Operations Research, the science that uses mathematical models to improve business operations. Also known as Management Science, therefore sometimes called OR/MS. See also www.informs.org.

When we speak about operations mathematically, it means things like add, subtract, multiply, divide, squaring, etc. Remember: If it isn't a number it is probably an operation.

When you see something like ...

7 + (6 × 5² + 3)

... what part should you calculate first? 

Start at the left and go to the right? 

Or go from right to left?

Warning: Calculate them in the wrong order, and you will get a wrong answer !

So, long ago people agreed to follow rules when doing calculations, and they are:
Do things in Parentheses First. Example:

		6 × (5 + 3)	=	6 × 8	=	48
		6 × (5 + 3)	=	30 + 3	=	33	(wrong)

Exponents (Powers, Roots) before Multiply, Divide, Add or Subtract. Example:

		5 × 22	=	5 × 4	=	20
		5 × 22	=	102	=	100	(wrong)

Multiply or Divide before you Add or Subtract. Example:

		2 + 5 × 3	=	2 + 15	=	17
		2 + 5 × 3	=	7 × 3	=	21	(wrong)

Otherwise just go left to right. Example:

		30 ÷ 5 × 3	=	6 × 3	=	18
		30 ÷ 5 × 3	=	30 ÷ 15	=	2	(wrong)

How Do I Remember It All ... ? PEMDAS !

P	Parentheses first
E	Exponents (ie Powers and Square Roots, etc.)
MD	Multiplication and Division (left-to-right)
AS	Addition and Subtraction (left-to-right)

source: mathisfun.com