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From Odwiki

Constraints are used to extract certain transform information from the surrounding objects and apply that to the transforms of other objects

Constraints found in other software and their equivalent in Houdini.

• Orientation constraint
  • The Blend Object is the solution to many constraint problems
  • In the help of the PreTransformCHOP it says "A combination of ObjectCHOP, PreTransformCHOP, and Export CHOPs can be used to perform orientation constraints in objects.
• Translation or point constraint
  • Blend Object saves the day again
• Two or three point constraints
  • Blend Object saves the day again
• Aim or Look-At? constraint
  • This constraint can be accessed at the transform tab of the object's parameters. The up vector leaves a bit to be desired. The up-vector can be set to aim at an object by using a expression to give the target object - the looking object's position. This also can be done in chops for more speed than the expressions
  • When you need an aiming up-vector it is better to use the Blend Object to make a Look-at the info on the Blend Object page will tell you how
• Path constraint
  • This constraint can be accessed at object level in the transform tab.
• InverseKinematics or IK
  • Can be created when you create a bone chain.
  • Can be added to the bone chain after creating the bone chain.
  • … And more stuff... maybe i will finish this later.
• Forward Kinematics or FK
  • can be just a hierarchy or can be added like IK in the viewport.
• SplineIK
  • Can be created when you create a bone chain.
  • Can be added to the bone chain after creating the bone chain.
  • The SplineIK can twist if you have a "twist" atrabute on the curve used to define the SplineIK. An example of this can be seen if you create a path object in the viewport, look inside any of the path cv objects.
  • In Houdini, the bone chain automatically solves to follow the normals of PathCV objects automatically if they are binding to a curve created by the object Path tool.
• CHOPs can do most other tricks including most of the ones listed here through creative use of the ObjectCHOP along with the CompositeCHOP (possibly among others).
• The FetchObject gives you the ability to parent an object in any hierarchy by giving it's path.

Anybody else have any thing else to add?