Boy, this is a tough one.
The snatch is a pretty complex lift, and it can be very difficult to get correct. This is one that really needs coaching.
I started to write a description / instructions, but after I got to about the 6th paragraph, I realized I wasn't going to do it justice (and heck, I'm not a coach . . . I don't even know if I'm doing right myself), so I figured I better stop.
Very simply, the snatch requires lifting a bar from ground to straight arms overhead, in one continuous motion. It requires "dropping under" the weight as it rises, so you end up catching it (at arms length overhead) in a crouch or split-leg position. Most people use a pretty wide grip, and the bar has to stay VERY close to your body as it rises, even skimming your shins.
You should always start to learn the motion with the lightest possible weight (i.e., a broomstick, not even an unloaded bar). You need to be pretty flexible in hips, knees, and ankles . . . able to squat comfortably with your feet flat on the floor.
That said, no description in words (that I can give) is going to be adequate . . . you have to see it done, and then you really need some good instruction to get the nuances right.
Coach is a huge fan of the World Class Coaching videotapes, available here:
Mine haven't arrived yet, so I can't speak from experience, but I expect they're pretty good.
Another video example I've found on the Web is here
You can also use the excellent "search" function on CrossFit to find the dozens and dozens of posts regarding the snatch . . . there's some good discussion of technique there.
Or, you can put "snatch" and "+Olympic lifting" into Google and come up with 10 screens worth of stuff (I'd definitely recommend adding the "+Olympic lifting" part . . . . "snatch" alone will focus on a movie of that name (and God knows what else)).