A rhombus is a kind of quadrilateral that has two sides that are parallel to each other, and another two sides that are parallel to each other. A rhombus is a special kind of parallelogram, and squares are a special kind of rhombus.
To figure out the perimeter of the rhombus, you can add the lengths of all the sides together. To figure out the area of a rhombus, you have to cut it into shapes that are easier to figure out. You want to cut it into a rectangle and two right triangles. Draw a line from one angle so that it is perpendicular to the opposite side, and another line from the opposite angle that is parallel to your first line, and also perpendicular to the opposite side. This will make a rectangle and two identical right triangles (one of them upside-down).
The area of the rectangle is the height multiplied by the width. The areas of the right triangles are the height multiplied by the width, divided by two. Add the areas of the triangles and the rectangle together, and you've got the area of your rhombus. (Because the two triangles are the same, you can just multiply the height by the width once and then add that to the area of the rectangle).
To find out more about math, check out these books from Amazon.com or from your library: