Disney's marketing department mixes and matches different deals on their tickets, depending on the season and its own particular calculations. If you check Disney's official Website, you can see which ones are in effect during the times you wish to go. Disney often offers discounts on multi-day passes--giving you 7 days for the price of 4, for instance--as well as "2fer" passes allowing you to visit two separate parks for the price of one. (Otherwise, the individual parks--4 at Disney World and 2 at Disneyland--each require separate admission to enter.) More whimsical deals include free admission on your birthday (bring your driver's license) and annual passports allowing frequent visitors to come as often as they like for a set fee.
As much as we may wish otherwise, there are plenty of people out there willing to swindle you for a few bucks, and Disney tickets make for tempting bait. In Florida, people are forbidden by law to sell used multi-day tickets to the Disney parks; beware of anyone trying to do so. Third-party sellers in general are not to be trusted, especially if they offer deals too good to be true. Beware of ticket sellers on eBay, or those who originate in Eastern Europe (where online scams involving Disney tickets have flourished). Authorized, professional Disney ticket sellers may not use "Disney" in their web address; if someone does, it's a good bet that they're not on the up and up. If you don't purchase your tickets directly from Disney, do so through a reputable travel agent with a longstanding track record that you can check.
Both Disneyland and Walt Disney World constitute multiple parks--two in California, four in Florida. Each park requires separate admission to enter. Most multi-day tickets let you visit multiple parks, but you can only visit one park on a given day with them. Disney sells "Park Hopper" options on their tickets for an additional fee, which lets you visit multiple parks in a single day. Generally speaking, however, they're not ideal, especially at Disney World. It requires a lengthy drive to move between parks in Orlando--15 minutes or so in most cases--and the whole process of moving to and from can take up even more time. Park Hopper tickets make more sense at Disneyland, where the two parks are about 100 yards apart, but every Disney park has more than enough to occupy your attention for a full day. Use the Park Hopper option only if you have a short amount of time.
