The Human Skeleton

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There are three main functions of the skeleton:

Support: The skeleton is what holds your body up right, without it we would be jelly!
Protection: The skeleton protects your inner organs for example your rib cage protects a lot of your internal organs such as the heart, liver, lungs, stomach and kidneys.
Movement: The skeleton allows you to do complex movements such as walking, running and lifting objects (along with your muscles).

The human skeleton is mainly made up of long bones as well as cartilage and various other different bones. below is a picture of the main structure of a typical long bone:

Blood Vessels: The bone contains blood vessels as it needs blood to stop it from dying as it is alive, also the bone marrow creates new red blood cells thus it needs to pass the new blood cells into the blood stream.
Bone Marrow: The bone marrow is in the inside the middle of the bone, it is what creates the new red blood cells as they cannot self replicate as they lack a nucleus.
Hard Bone: Hard bone is what gives the bone structure and support as well as protecting the bone marrow.
Cartilage: Cartilage is very important, it is what stops your bones from rubbing and smashing together. Cartilage is found at the head of a bone where the joints are to absorb shock. If there wasn't any cartilage to absorb the shock then your bones, even from the smallest bit of shock forced upon the bones, could receive extreme harm and potential cracking or fracturing. Cartilage can get worn away quite quickly and the older your are the less quickly it regrows.
Air Pockets: Air pockets keep the bone light and flexible, without air pockets you would weigh significantly more and your bones wouldn't be able to take as much stress/strain.