Simple carbohydrates are easily digested releasing energy and raising blood sugar quickly. This is good if you are about to do hard exercise, but not if you are sitting at the office or in front of the TV, in which case it is stored as fat.
Complex carbohydrates require the body to work harder to digest, releasing energy and raising blood sugar more steadily. Eating too many carbs can cause obesity and type 2 diabetes. Carbs should make up no more than 1/4 of your meal. Wherever possible, choose complex carbs, especially those high in fibre. 
Below is a list of complex vs simple carbs: