Could someone describe a little bit about linear bearings. I need to build my own for my CNC and would like to know as mcuh about them as possible before I begin designing.

Surf to the Linear Motion forum (http://www.cnczone.com/forums/forumdisplay.php?f=279), read some of the threads and you shall find. :)

Well, you request is a tall order - there is a lot to know about linear bearing and even more to know about how to apply them.

Linear bearings provide stiff linear guiding with limited friction. There are bearings with rolling elements and there are bearings which have no moving parts.

Scott

these are two most popular designs:

"THK" style: http://www.thk.com/images/ja/products/class/lmguide/lmguide_00.jpg (i preffer these becouse they are easier to work with, at least for me)

and round bearings(not sure of nickname for these): http://www.advancedbearings.com/images/Thomson/SuperSmartBBb.jpg

they use shaft as rails and then those bearings.

Also there are quite few designs that use skate bearings. Most popular use gas pipe as rail:
http://www.cnczone.com/forums/attachment.php?attachmentid=12258&d=1132911833

some use angle as rail, and some use other kind of material as rail: square tubing, C channel, etc


Best thing to do if you are planing to buy used rails is surf ebay and see what fits your wallet