Much of this information you can find at the Nasa Great Lakes site: http://www.nasagreatlakes.com
At Road America (and all Nasa Great Lakes events) we had the following groups:
HPDE1: Start for hpde, all students will have an in car instructor for all of their sessions.
HPDE2: Student has mastered HPDE1 skills and will have an instructor for some of their sessions and solo for the others.
HPDE3: Student has mastered HPDE2 skills and will drive solo in a fast run group with liberal passing, comfortable with driving offline and side by side.
HPDE4: Top of the HPDE groups with open passing and preparation to go into Time Trials or Comp school.
Time Trials: competition for fastest lap in your run group. Requires advancement to HPDE4 to get a provisional Time Trial license, once that is passed you receive a time trail license that will allow you to compete in Time Trials in any region.
Thunder and Lightning: These are the race groups. Advance to HPDE4 (or TT) and then attend Competition School to receive a provisional race license. You can only run in that region until you complete 4 events without incident at which time you can apply for a national race license.
As for the 'Levels' of racing and some of the reason for cars trailing the pack is that there are many classes out there racing together. Thunder usually has more horsepower than lightning cars but there is a big difference between the abilities of the fastest cars in the group to the slowest. Time trials has only one group so the difference is even larger.
The web site also contains links for cars that can be rented at http://www.nasagreatlakes.com/GetStarte ... ntals.aspx
The next event is June 25-26 at Gingerman and registration is at https://www.nasaproracing.com/events/2177
. If you are not sure which HPDE group you should sign up for, there is a wizard on the Great Lakes site that will ask you about your experience and suggest an appropriate class.