OpenGL Question

I was wondering if someone could clear up a few questions I've got about OpenGL, or at least point me in the right direction.
First - what's with the different API versions 1.4, 1.5, 2.0, 2.1
I'm under the assumption that you would wan the latest version?
Second - How would you go about getting it? (SDK?)
It's just libs and header files if I'm correct so is there a place to download
them (the/a SDK)?
Lastly - besides GLUT and GLU are there other toolkits/extensions that are
commonly used?

I've spent quite a bit of time scouring the web (including the OGL and SiliconGraphics websites) trying to understand this but . . .
I know these question may seem ridiculous to some of you, I just wanted some clarification and needed to know if OpenGL is wort my while. Thanks.

The different API versions just specify the different specifications, generally denoting which ARB extensions have been promoted to the status of 'core' functions.

Windows currently only ships with libs and headers for OpenGL 1.1. That means to get access to newer features, you need to use them as extensions, even if they've been promoted to core in later versions. For example, all GLSL functions were promoted to core in OpenGL 2.0, but still need to be accessed as extensions when using the windows libs. Hopefully Vista will change this. Or may the Khronos Group.

You may want to have a look at GLEW (OpenGL Extension Wrangler), which is a nice lib for dealing with those nasty extensions.

EDIT: Here's a nice article on the matter.