Your lap is a place that only exists when you sit down. It is the place on the top of your thighs when you have your knees lifted up. The boy in the photo has a cat on his lap. A lap is also a full circuit of something - for example, you may run a lap around the school oval.
You have two legs, one on each side. You use them to stand on, and to run and jump. Your feet are at the ends of each of your legs.
Limbs are your arms and legs. A bird's wings are called limbs, and large branches on a tree are also called limbs.