Expressions

Fractions can be undefined. An undefined fraction is a fraction that doesn't make sense.  Simply: an undefined fraction is a fraction with a zero denominator. 

Let's think about it:

If a have a pizza and cut it into 6 pieces, I can give you 1 piece ($\dfrac{1}{6}$) or 4 pieces ($\dfrac{4}{6}$).

Because rational expressions are just fractions with variables, the process of simplifying rational expressions is conceptually the same as the process of dividing numeric fractions. You can divide the top and the bottom of the rational expression by the same number. Because dividing the numerator and denominator by the same number is the same as dividing by 1, it leaves you with an equivalent fraction.

Expressions are different from equations in that they don't have equal signs. The contain variables, but because they are not equal to anything, you can't solve for the variable.

However, sometimes you will be asked to evaluate expressions when you are given the value of the variable (or several variables).