Let’s start off with the similarities:

Both are resorts with multiple theme parks:

Walt Disney World has the Magic Kingdom, Epcot, Disney’s Hollywood Studios, and Disney’s Animal Kingdom. Universal Orlando Resort has Universal Studios Orlando, Islands of Adventure, and soon to open Volcano Bay. Many people get confused because both resorts have a studio-themed park.

At both Disney and Universal you can buy separate tickets for the individual theme parks or get a pass that allows you to move freely between them. This is called a Park Hopper pass at Walt Disney World, while it’s simply a two or three park ticket at Universal.

Both Disney and Universal have on-site hotels with free transportation to their parks:

They also have entertainment districts, with Disney Springs at Walt Disney World and CityWalk at Universal. Those districts both have restaurants, shops, clubs, and movie theater complexes. CityWalk also has a miniature golf course, while the mini golf courses at Walt Disney World aren’t located at Disney Springs. It does, however, have bowling, tethered balloon rides, boat rentals, kiddie rides, and a few other offerings that you won’t find at CityWalk.

The Ticketing system- Universal’s system is called Express Pass, and it’s only free to guests staying at Universal hotels. Other guests must buy an Express Pass if they want to take advantage of the shorter lines. Unlike FastPass, which has limitations, the hotel based Express Pass at Universal is unlimited. The standard purchased version lets you use the Express line once on each ride, or you can pay more for unlimited Express Passes.

And now the differences:

You won’t find the Wizarding World of Harry Potter at Walt Disney World. Because Disney is the big player in Orlando, many tourists just assume that it’s Harry’s home base. Hogwarts Castle and Hogsmeade are actually at Islands of Adventure. Some guests think the Harry Potter area requires a separate admission ticket, but it’s included with regular Islands of Adventure admission or your Universal two-park ticket.
You won’t find SpongeBob, the Minions, Scooby Doo, the Simpsons, the Cat in the Hat, the Madagascar gang, or even SpiderMan and other Marvel heroes at Walt Disney World. Universal Orlando Resort has these characters in its parks, while you’ll find Disney/Pixar characters in the Disney parks. You might say, “But Disney owns Marvel!” True, but Universal had a long-term agreement in place before the sale, so Spidey and the other Marvel superheroes can only be found at Islands of Adventure.

Since the two theme parks are competitors their is no shared transportation between the  Universal parks and the Disney World parks. You can, however, take an independent shuttle to Universal if you’re staying at a Disney-owned hotel, and you can get to Disney from a Universal hotel in the same way.