In our area we see people start out in IDPA and once they get comfortable with action shooting in front of the crowd, they then sample USPSA. They are superficially similar but also quite different.
A post on this forum mentioned that he was not used to longer shots. This is one of my pet peeves...if you only shoot 7 yards or less you'll give up lots of points in IDPA or USPSA.
There are other shooting sports such as ICORE(only revolvers), Steel Challenge(speed shooting steel targets) and pin shooting(hard to find).