The 321 kinematic structure is a robot kinematic structure most commercial serial manipulators have. The inverse kinematics of serial manipulators with six revolute joints, and with three consecutive joints intersecting, can be solved in closed form, i.e. analytically. The 321 design is an example of a 6R wrist-partitioned manipulator: the three wrist joints intersect, the two shoulder and elbow joints are parallel, the first joint orthogonally intersects the first shoulder joint.
Many other industrial robots, such as for example the PUMA, have a kinematic structure that deviates a little bit from the 321 structure. The offsets move the singular positions of the robot away from places in the workspace where they are likely to cause problems.