A Camelbak is the original NATO issue water bladder.
Issued to Brit forces as woodland or desert DPM, incorporating the 'coolmax' system. It is essentially a plastic bag that magically keeps the water (or other 'beverage') cold. A pain in the arrse to fill up but bloody useful.
The delivery tube uses a bite and suck nozzle... a bit like your mum!
While any walt can and indeed will buy his own for full price, a good squaddy (is there any other kind?) will get a decent one off eBay. A normal squaddy, of course, just gets one on issue ...
Woodland pattern NSN:8465-99-842-7034 Desert pattern NSN:8465-99-687-2284 (although this may just be for the cover!)