Ghosting not only makes an object invisible when rendered , but also works
hand in hand with raytracing to create some very nice render effects.
Objects that are ghosted can still interact with raytracing. They will still
show up though transparent objects and be reflected on objects with reflection turned up.

To ghost or unghost an object right click on it and select ghost or unghost
on the menu depending on the objects current status. Or right click on the objects name
and do the same. Ghosting can also be done on the project list by selecting the first white ball
next to the objects name.

To make an object render completely invisible, raytracing needs to be turned
off on the object.

To turn off raytracing on an object right click on it and select properties
or right click on the objects name and do the same.
Or on the project list by selecting the greenish ball
next to the objects name will bring up the object's properties box.
On the render tab Raytrace Interaction is turned on by default.
To turn it off remove the check from it's name.

Here are some examples of ghosted and not ghosted objects
and how they interact with raytracing.

Objects will reflect ghosted objects

A normal sphere being reflected on a plane.

The sphere has been ghosted.

Sphere ghosted raytracing turned off.