Let's Make Robots!

selecting servo's?

Hi,

I am about to build my first real robot, based on a partially gutted RAD robot.  The electronics are no problem to me, I have a good understanding of what is required there.  My problem is that I completely suck at anything mechanical.  I have been looking at servo's to move it's head, sensors, arms, etc but I cant figure out what the difference is between them, and how to select the ones best for my application.

Thanks in advance.