So what's a Browser Helper Object (BHO)?

In a nutshell, a Browser Helper Object (or BHO) is a small piece of software that "helps" your web browser by adding additional functionality or features.  As in most areas of life, there are the good and the bad.  Examples of helpful, safe BHOs include:  x, y, z.

Among the bad guys are:  1, 2, and 3. 

What makes a bad BHO "bad"?  A Browser Helper Object is bad when it hides, does anything without your permission or hijacks your browser in some way.