Area and perimeter
Perimeter is the distance around the edge; area is the surface inside it.
A worked example
A rectangle 8 cm by 3 cm has perimeter 8 + 3 + 8 + 3 = 22 cm, and area 8 x 3 = 24 cm².
The mistake people actually make
Perimeter is measured in cm, area in cm². If your units do not have a square on them, you have found a perimeter.