Potential is the gravity that can be achieved with 1lb of malt mashed in 1 gallon of water.
A list of malts and their potential can be found at the following link.
Yeast attenuation is the percentage of the sugar that a strain of yeast will consume.
If you don't know the attenuation of your yeast strain, it can be found on your yeast distributors website. A few of the most common brands has been linked below.