From BlenderWiki

Rigid Body Joint Constraint

Description

The Rigid Body Joint constraint is very special. Basically, it is used by the physical part of the Blender Game Engine to simulate a joint between its owner and its target, either hinge type, ball-and-socket type, or generic six-DoF (degrees of freedom) type.

The joint point and axes are defined relative to the owner, and are fixed relative to it. The target moves as if it was stuck to a stick, at its center point, the other end of the stick rotating around the joint/pivot point…

So this constraint is of no use in most “standard” static or animated projects. However, you can use its results outside of the BGE, through the Game » Record Game Physics to IPO menu entry (from the main menu of the User Preferences window, see this page for more info on this topic).

Options

Targets

This constraint uses one target, and is not functional (red state) when it has none. Note that the target field is called toObject here…

Joint Types

Ball

works like an ideal ball-and-socket joint, i.e. allows rotations around all axes.

Hinge

works in one plane, like an elbow: the owner and target can only rotate around the X axis of the pivot (joint point).

Generic 6DOF

The Rigid Body Joint constraint panel, Generic 6DOF joint type.

works as the Ball option, but the target is no more constrained at a fixed distance from the pivot point, by default (hence the six degrees of freedom: rotation and translation around/along the three axes).

In fact, there is no more joint by default, with this option, but it enables additional settings which allow you to restrict some of these DoF:

LinMin and LinMax

When these options are enabled for a given axis (of the pivot), the allowed variation along that axis of the distance between the target and the pivot point is limited by the corresponding values, in Blender Units.

AngMin, AngMax

When these options are enabled for a given axis (of the pivot), the allowed rotation around that axis is limited by the corresponding values, in degrees.

No Collision

When enabled, this will disable the collision detection between the owner and the target (in the physical engine of the BGE).

ShowPivot

When enabled, this will draw the pivot of the joint, in the 3D views. Most useful, especially with the Generic 6DOF joint type!

Pivot X, Pivot Y, Pivot Z

These three numeric fields allow you to relocate the pivot point, in the owner’s space.

Ax X, Ax Y, Ax Z

These three numeric fields allow you to rotate the pivot point, in the owner’s space.

Influence

This constraint has no Influence setting, nor the related Show and Key buttons, as it is only functional in the BGE.