It's been a few years since I bought a Lego Mindstorms (v2) which comes with the Technic style pieces. 

There's beams, axles, rods, connectors... but the majority of the times when I'm building (my own designs, not following instructions) I wonder which is the right part to use. There's usually several options and I never found what is the exact intended purpose of each one.

Is there a website, document, or anything that I could use as a reference for what each part in the Technic set is meant to do?