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
