Structural joints are the connections between different parts of a structure, designed to hold them together and transfer loads. They can be classified based on their function (e.g., expansion joints, hinge joints) or their structure (e.g., fibrous joints, cartilaginous joints, synovial joints). Properly designed joints are crucial for structural integrity and stability. 

Bridge expansion joints accommodate movement caused by thermal expansion and contraction, and other factors like traffic loads. They are classified by the amount of movement they can handle (small, medium, large) and by their design. Common types include strip seal, finger joints, modular joints, sliding plate joints, and elastomeric joints.