Botox contains a neurotoxin produced by Clostridium botulinum. This toxin prevents the release of acetylcholine in neuromuscular junctions. Neurons release this chemical signal in order to make muscles contract. Blocking that signal also blocks muscle contraction. It's more or less the inverse of nerve agents such as sarin.

As with all toxins, the dosage makes the poison. It's deadly yet has medical applications. Can be used to treat muscle spasms, neuropathic pain... Most famous for its cosmetic applications though: it relaxes the muscles of the face, removing wrinkles. I know doctors who literally live off of this stuff.