QUESTION:
What is the chemical composition of hydrazine and what physical properties
make it the chemical of choice for propulsion?

ANSWER from Chris Salvo on July 27, 1997:
Hydrazine is N2H4 (the numbers should be subscripts).  It has a number of
properties that make it attractive.  It is a monopropellant, which means
given the right catalyst or conditions it can be made to burn all by itself. 
This simplifies the spacecraft design (only one type of propellant - no
mixing to worry about, less plumbing, etc.).  Hydrazine is also stable and
relatively safe at room temperature and pressure.  It is storable in liquid
form for long periods of time.

The bottom line is that hydrazine is cheap for a given performance
requirement.
     
There are probably other advantages, but I am certainly not an expert on the
subject.
     
Chris Salvo