The Limiting Reactant is the reactant whose amount limits the amount of product formed.

Finding which reactant is the limiting one:
1. Find the amount (#moles) of reach reactant
2. Divide this number by the coefficient of the reactant that appears in the reaction
3. The smallest quotient you get is the limiting reactant

Use the limiting reactant to find the product in a stoich problem when they don't give you a reactant to start with =)