How are expressions and equations different ?

The primary difference between equations and expressions is equal signs. An "equation" has a left side, a right side and an equals sign separating the sides. An expression however, we are not expected to do or solve anything.

For example:

3x - 7 = 2

This is an EQUATION, because it has a left side, a right side, and an = sign separating the two.

3x - 7

This is an EXPRESSION, because there are no "sides" and no = sign

6mm^3 or .6cm^3

      216

      /   \

  36     6

  /   \     /

6     6   6

factor trees are nice.