Activity Monitor questions

Ok, just trying to get to know my machine better...

I have 1 GB of RAM installed on my Intel iMac, and I was just wondering what all the different things mean on the System Memory tab on Activity Monitor. I kinda know what Page ins/outs means (the higher the page outs, the more I'm having to use slower, virtual memory, but that's about all I know), and I can guess what the green Free means (RAM not being used, duh). But what's the difference between the red, yellow, and blue (Wired, Active, Inactive). It seems to me that if it's inactive, it should be free, but I don't know too much about this kinda stuff.

And while I'm at it, what does "Nice" mean on the CPU panel?