Charm is persuasion by feeling. The character does not argue a case or issue an order; they make the other party want to agree — through warmth, wit, flattery, attention, desire, or simple presence. It leans on how a person looks and on how well they read the mood in front of them, which is why it is at once the most natural of the persuasion skills and the least reliable: what works on one listener is exactly what closes another off.
Charm is also the skill of seduction, and of the first impression that decides a room before anyone has said anything of substance.
Resistance. One does not resist inspiration as such, but a target may actively oppose seduction with a Will test, or work out what is being done to them with Intrigue or Empathy. Empathy alone reads the feeling — that the charmer is warm, or nervous, or intent — while Intrigue is what identifies the purpose behind it.