They will give you cash, or if they are low on cash or otherwise at their discretion they may give you a paper check for the amount instead.

I've received a check instead of cash once when they were low on cash.

I always go to the customer service counter to cash my reward cert but I believe any checkout can do it as well. Makes more sense to get the cash instead of using for a purchase so you don't miss out on rewards for that purchase.

At this point I pretty much only use my Costco Visa for Costco gas purchases. It makes more sense for me to use my Wells Fargo Autograph for travel and transit (if not using my CSR) and my WF Active Cash at Costco for 2% back since WF's rewards don't require a trip to a store to redeem..