Parkinsons disease is characterized by the death of cells that produce dopamine, a neurotransmitter. An enzyme called monoamine oxidase (MAO) breaks down neurotransmitters.  MAO has two forms, MAO-A and MAO-B.  MAO-B is generally believed to break down  dopamine; however, recent evidence suggests that MAO-A may mostly or entirely be responsible for dopamine metabolism. Rasagiline prevents the breakdown of dopamine by irreversibly binding to MAO-B. Dopamine is therefore more available, somewhat compensating for the diminished quantities made in the brains of people with Parkinsons.
Selegiline was the first selective MAO-B inhibitor.  It is partly metabolized to levomethamphetamine (l-methamphetamine), one of the two enantiomers of methamphetamine, in vivo. While these metabolites may contribute to selegilines ability to inhibit reuptake of the neurotransmitters dopamine and norepinephrine, they have also been associated with orthostatic hypotension and hallucinations in some people. Rasagiline metabolizes into 1(R')-aminoindan which has no amphetamine-like characteristics and has neuroprotective properties in cells and in animal models.
It is selective for MAO type B over type A by a factor of fourteen.