A WeldJoint constrains all relative motion between two bodies.
A revolute joint rains two bodies to share a common point while they are free to rotate about the point.
A fixed revolute joint rains a body to share a common point with the word while it is free to rotate about the point.
An angle joint maintains a fixed angle between two bodies.
An fixed angle joint maintains a fixed angle on one body
A line joint provides two degrees of freedom: translation along an axis fixed in body1 and rotation in the plane.