A saddle joint is a synovial joint that allows side-to-side and back-and-forth movement with small amounts of rotation. The most obvious example in the human body is the joint at the base of the thumb.
All synovial joints contain cartilage and fluid to allow for better movement. Saddle joints consist of two bones with both a concave and convex component. The bones fit perfectly within one another, similar to the legs of a rider on either side of the saddle fitting in the seat created between the front and back of the saddle. The movement allowed by a saddle joint gives humans the ability to grip items.