Can anyone point me to a thread or a link where I can do a little layman-friendly research about phobias?

I've just been wondering what causes them. I suppose some might be due to a childhood experience, and I guess most phobias relate to something of which it would be reasonable to be afraid (snakes, spiders, thunderstorms, etc.)

Just curious at what point severe anxiety or fear about someting crosses the line and becomes a true phobia, and why some people develop them and some don't.

I always wonder why the next person can relax and sleep through a closed MRI, whereas I get anxious and short of breath just THINKING about somebody ELSE in that tube.

Anybody point me toward some answers? Thanks!