DVC is Disney's version of a timeshare. To become a member, you purchase a real estate interest in one of the DVC resorts. More information is available in the FAQs on the DVC boards, if you're interested in joining.
If you're not DVC, you may only use the pools at the resort where you're staying, with a couple of exceptions:
1. If you're staying at POR or POFQ, you can use the pools at both resorts.
2. If you're staying at an All-Star resort, you may use the pools at all 3 All-Star resorts.
At YC/BC/BCV, the pool area is fenced, and you must show a YC/BC/BCV resort ID to get in.
At the other deluxes, lifeguards and/or security conduct random resort ID checks. They walk around the pool area and ask to see your ID. If you can't produce an ID for that resort, you'll be asked to leave.