Botox®, or Botulinum toxin A, is a protein derived from the bacteria Clostridium botulinum. It works by blocking the signal between the last part of the nerve and the muscle that is being targeted, causing the muscle to relax and therefore not function. As the nerve fibres regrow, the function slowly returns.
In fact, Botulinum toxin was first discovered to treat wrinkles as a side-effect of migraine treatments using the product. It has several other medical uses including prevention of excessive sweating (hyperhidrosis) and dystonia (muscular tics and spasms). It is of course best known for being able to reduce wrinkles!
Botox is a brand name and there are other brands such as Dysport and Boccoture. I mainly use Bocouture and achieve exceptional, natural looking results.