Just to clarify, this dude will have his day in court, and I suspect he'll be facing some serious time (for manslaughter at least). Obviously, I could be wrong.
Based on what I've read, this is what we can be somewhat confident about.
Martin, a 6'3'', 140 lb 17-year-old African American male was walking home from a convenience store while visiting his dad in Florida.
Zimmerman, a 6'3'' (?240 lb?) 28-year-old Caucasian, Hispanic male was patrolling this (his) neighborhood, spotted Martin, and began to follow him.
In some order (I'm unsure):
Martin started walking faster and pulled his hood over his head; Zimmerman called 911 to report a suspicious individual in his neighborhood, referring to Martin as both an ******* and a ****ing coon.
The 911 operator told Zimmerman that his assistance was not needed.
Zimmerman continued to follow Martin.
Martin ran and tried to get away.
Zimmerman exited his vehicle with a gun, and confronted Martin.