Throughout history the notion of a person turning into wolf has haunted the edge of our stories. Barry Holstun Lopez in 'of Wolves and Men' speculates on how early man wanted to emulate the wolf as hunter. His book vacillates between the mystic and the poetic (and occasionally both at the same time).
In 1865 the prolific writer and folklorist (now he would be called a cultural anthropologist) Sabine Baring-Gould wrote "the Book of Werewolves" - a collection of fables and histories that is still regarded as a standard reference on the subject. He touches briefly on the various forms.
He traces one of the earliest werewolf stories to Ovid's Metamorphosis (see Arachne) tale of King Lycaon of Arcadia; being honored by a visit from the King of the Gods (Zeus or Jupiter) set before him his son in a stew. Zeus was outraged by the human sacrifice and turned the King into a wolf.
The quasi-historian Pilny notes that the Arcadians of his time yearly selected a man from a family that claimed descent from King Lycaon and in the spring (coincidentally at lambing time) the man was required to swim across a boundary river, leave his clothes behind and put on a wolf skin. He supposedly became a 'wolf' and ran into the woods to protect the flocks. After nine years, if he had not tasted human flesh, he could swim back and "become a man" again. (Interestingly, Romanian legend says that if a vampire does not drink human blood for seven years, they regain the ability to walk in the sunlight and live as a "normal" human.)
In the Jim Butcher novel 'Fool Moon' Harry Dresden's magical consultant Bob says of France during the Inquisition:
". . . We had every kind of werewolf you could think of. Hexenwolves, werewolves, lycanthropes and loup-garou to boot. Every type of lupine theriomorph you can think of."
"Theriomorph" he goes on to explain is any human who changes into an animal.
What we call the "werewolf" is a human being who uses magic to change themselves into a wolf. It turns out to be the most helpless of the changes because the wolf retains the memories and personality of a human and humans have to learn to deal with the sensory bombardment of the wolf - not to mention learning how to walk on all fours. Any weapon that can harm a true wolf can kill a werewolf.
The hexenwolf comes from another legend. A pact is made with a wizard, witch or demon and the one who makes the pact is given a talisman - usually a belt of 'hairy wolf's hide'. The belt contains a "spirit of rage" that gives the hexenwolf the shape and abilities of the wolf while retaining human intellect. Clearly the most dangerous form of the monster but they are in danger themselves, because slowly the wolf takes over - the animal rage is an adrenalin high that addicts its victim until they can no longer turn back and become "true wolves." Again, any weapon that can harm a true wolf can harm them and in human form the Inquisition burned them at the stake.
The lycanthrope is a psychological condition where the person takes refuge in acting like an animal. In other stories they are human who have a spirit of rage inside them. In times of stress (like the Norse bersark also known as the berserker) or at the full moon they become aggressive. They seem to be very resistant to pain and injury - they are born this way. It is suspected that ancient serial murderers were accused of being such "human wolves."
The most dangerous lupine theriomorph is "the loup-garou." This is a human that has been cursed by someone with great power (Bob mentions the Fairy Queen); when the full moon comes they go on a slaughtering spree until the sun rises. They are mindless killing machines with supernatural speed and power. They recover from injuries almost instantly and are immune to any and all poisons (think Wolverine, true believers). They are the only form that can be hurt by silver - and not just any silver; it must be heirloom silver - inherited from a family member.
This curse can be passed down in families- usually the individual is "marked" by being born on midnight of the Winter Solstice, or being born on Christmas day, or being born with red hair, having extra fingers or toes, having pointed ears. The unfortunate is aware of their condition and takes great care to lock themselves away.
Modern werewolves were altered by Hollywood for dramatic purposes - the lines of the hand must form a star (because actor Michael Landon's hand was so marked and the writers of "I was a Teenage Werewolf" thought it was interesting) or a person can become a werewolf by being bitten by a werewolf (as in the Lon Chaney classic).
In the 1990's a Role-Playing Game companion to "Vampire: the Masquerade" was written; "Werewolf: the Apocalypse" changes the rules yet again, speculating on werewolves born of ancient blood, able to control their changes at will and eternally in battle with Vampires and Humans.
Werewolves are grimly dangerous monsters and not to be trifled with.