The "original" Sith were a humanoid race found by the first dark Jedi exiled by the Jedi Order. They were enslaved by the dark Jedi. They were powerful in the Force, and for that reason the dark Jedi interbreeded with them, creating a race of even stronger dark Jedi, which would eventually become known as the Sith Empire. There was also a difference pre-Bane and post-Bane:
Pre-Bane Sith Empire Caste System:
Dark Lord of the Sith - usually the strongest and most powerful Sith, at certain times there was more than one
Sith Lord - Members of the Sith Council, lead many of the lower ranks into battle
Sith Warrior - mainly soldiers, with concentration on strength in the Force and fightining ability
Sith Acolyte - Force-sensitive who has begun his Dark Side training, similar to a Jedi Padawan
Sith Minion - usually non-Force-sensitive beings who served the Sith
Post-Bane Sith Empire Caste System:
Sith Master: Usually the stronger of the two Sith, he would train his apprentice until he/she was strong enough to fight their master and kill him/her
Sith Apprentice: Usually the weaker, remained the student of the Sith Master until strong enough to kill him/her, after which he/she would become a Sith Master and search for a new apprentice
It is unknown what the original Sith looked like.
It is also unknown whether someone trained as a Jedi originally made a difference in their strength overall. Evidence supports the inverse.