Using the autorigs is an alternative to rigging your own character.
Ready-made auto rigs
There are two ready-made auto rigs on the Auto Rigs shelf. All you have to do is click the 
Biped Auto Rig or the 
Quadruped Auto Rig tool on the shelf and you are ready to animate.
See the biped autorig tutorial for how to use the autorig to rig a character.
There is also a 
Toon Character on the Auto Rigs shelf, which is animation-ready. It also comes with built-in deformation, facial shapes, and squash and stretch ability.
Build your own rig
Houdini gives you the option of creating your own rigged character by connecting body parts together. There are several appendages for both the biped and quadruped characters.
Simply create a character placer in the scene, and then add other body parts using the tools on the shelf. The following are some of the appendages you can use to build your own rig:
Note
There is also a tool called Reconnect Part on the Auto Rigs shelf that is useful if you ever select the wrong hook object or change your mind about where you want the body party to be connected.
Once you assemble your rig, you can select all of the body parts you want to make into a rig (including the character placer), and click the 
Create Auto Rig tool on the Auto Rigs shelf.