Flexibility is the ability to move joints through a maximum range of motion without undue strain. Flexibility really depends on the soft tissues (muscle, tendon, ligaments) of a joint rather than the bony structure. However, the structure of the bones in certain joints can place limitations on range of motion. For example, when lifting the leg out to the side, the leg will be able to move into a higher position if the hip is turned out. The reason for this is that the bony structure on the upper leg bone moves out of the way and allows the leg to move more.

Strength goes hand-in-hand with flexibility. For the example given above, it's clear that if you turn the hip out before trying to lift the leg, the leg is likely to lift higher. What may not be so evident is that in order to lift the leg there is a certain amount of strength needed to bring up the leg. This is referred to as functional flexibility. You have to have the strength to move the limbs through their range of motion, otherwise there is no use for the flexibility.