Mario Kart Wii/Time Trial and ghosts

The Time Trial mode pits you up against yourself, however you will be accompanied, at first by a staff ghost (often referred to by players as a "normal staff ghost").

By completing a time trial with a very low time you can unlock expert staff ghosts, additional characters, and additional karts.

What is a ghost?
Ghosts are transparent players that you can drive through. They cannot attack you, nor be attacked. The goal of the ghost is to give you a challenge to beat. Ghosts are also important guides, as they can show you ways of minimizing your time on a coarse. If you mimic some of their decisions, you can get the best times as well.

What is an "expert staff ghost"?
Expert Staff Ghosts are unlockable ghosts that have better times than the default, normal staff ghosts. An expert staff ghost can be unlocked by getting a good time on any time trial. Expert Staff Ghosts have to be unlocked on each stage. Once unlocked, they will replace the normal ghost. You cannot go back to the normal ghost once an expert staff ghost has been unlocked.