Left-Hand Path religions are ones that hold the physical to be as (or more) important as the spiritual. They are generally focused on balance, of body and mind, of light and dark, of good and bad.
LHP religions are fewer and less popular than RHP religions, possibly because of the diffusion of Christianity. Some of the most practiced LHP religions are forms of Satanism or Thelema.
Because of the 'balance' aspect of it, LHP practioners tend to be more open to the darker side of things, often practicing 'black' magick. They are also less acknowledging of taboos, treating nearly every decision as that of an individual. As such, they are often seen in a bad light.
Note: I wrote this myself, so I don't have any sources.