Monomethyl hydrazine is based on hydrazine, which has the chemical formula N2H4. Hydrazine is also used as a propellant and is both unstable and toxic. It may be used as monopropellant (usually after being run through an appropriate catalyst bed), or more commonly as a bipropellant, being hypergolic with nitrogen tetroxide (NTO). Applications of hydrazine have included propelling the Me163B (the first rocket-powered fighter, WWII), on the Viking and Phoenix lander descent engines, and powering F-16 emergency power units. Notably, the thrusters on the spy satellite USA 193 were fueled with hydrazine; an interesting article on the role of hydrazine in the satellite shootdown is here: http://spectrum.ieee.org/aerospace/satellites/us-satellite-shootdown-the-inside-story
Monomethyl hydrazine (MMH) is the result of replacing one of the hydrogens in hydrazine with a methyl group, CH3. It is more stable than hydrazine (can be used in regeneratively cooled engines), and is thus favored as a storable propellant. Because it is still toxic, as well as a suspected carcinogen, great care is taken in handling it. It is most commonly used in hypergolic combination with NTO, such as in the Space Shuttle’s Reaction Control System (thrusters). MMH decomposes into mainly H2, with some CH4, N2, and trace amounts of NH3 and C (soot).