From what I know, 1x1 is plain weave and 2x2 is twill weave, but I’ve seen some site has some plain weave 3k x 3k and 3k 12x12 (fiberglasssupply.com)?? 
I am just confused how do you read those symbols or numbers? what’s the k stand for?
3k is 3000 strands of fibers per tow.
Tow is each…umm…yarn thread, which is then weaved.
So, 2x2 twill is a weave patter. Under 2, over 2, etc
2x2twill 3k means that each tow of the weave has 3000 strands.
12k obviously means 12000, which is a much thicker tow, and much stronger.
Harness weave (4HS, 8HS) is also a weave.
Check out http://www.compositesworld.com/sb/ and http://www.compositesworld.com/ for some more general details.
That make sense to me now, Thanks.
One correction. The 12 x 12 in the above example is not the tow size. It is the pics per inch. 12 means there there are 12 fibers per inch in both the warp and fill directions.