There must be several thousand threads - both here at LQ as well as everywhere else on the web.
Short answer is that there is no short answer - shared memory, kernel uses, slab cache (no it's not included in the cache you see elsewhere) ...
That is a really old kernel, and by the looks of it only 32-bit. Years ago Linus said he would refuse to even talk to people who have that much memory on 32-bit. Let's see the output of these - use [code] tags around what you post