OK, I am trying to figure out this forza stuff. Today I figured out how to get rid of the ghost cars that are on the track when I am doing hot laps. What I would like to know is:
1) what are they
2) why are they part of the game
I asked the googles, and it appears that they are previous runs with my car, but runs from when? Is there any reason that should leave them on? What good are they to me?
The ghost is usually your best lap, either your best of all time or your best for that session. The usefulness is so you can see what technique is faster through a particular corner/section.
Dogbreath pretty much said it. The two ghost cars you'll see are your best laps. One for that session and one for your best lifetime in that class.
Thanks guys.
What if your fastest times are during solo hot laps instead of races in game play. Will they still show up as ghosts?
It doesn't matter when or how your fastest lap occurred: hotlap, time trial, or race. If you're going around a track and beat your previous best; that is your new ghost.
I shouldn't matter, you get faster every time you go around the track, so the ghosts should be behind you, right?
That would be the plan. 
They should be yes. When it gets annoying is when you are at the limit of your ability and are running laps consistently to within about a tenth. Then you get your car and the ghost merging in and out from each other and since the ghosts are only translucent it can get really aggravating with sight lines being blocked.
