Perimeter is the total distance around an object. There is a specific formula for perimeter of a rectangle, P = 2L + 2W, but for other shapes, you just add all the sides up. Because circles don't have sides that we can put a ruler to and measure, we have to find a different way to find the total distance around the circle. We call this, circumference. You can find circumference using 2 formulas: C = pi x diameter or C = 2 x pi x radius. The reason there are two formulas is because 2 radii = 1 diameter. You have to look at what the problem gives you. Remember, perimeter and circumference both measure the distance around the outside of an object!
Area is the total spaced covered inside an object. It's the flat space that something occupies. you must look at the shape and determine what it is so you can pick the right formula! The formulas can be seen below. Make sure you choose the right one to solve!
Extra Practice: http://www.mathgoodies.com/lessons/vol2/practice_unit2.html