Orlando is built for the car. The theme parks are spread across miles of suburban sprawl south and west of downtown, and distances between them are real — 20 minutes from International Drive to Disney, longer in traffic. A rental car is the easiest way to move around; most visitors pick one up at Orlando International Airport.
If you're staying on Disney or Universal property, the resorts run their own internal transport and shuttles to the parks, which can cover most of what you need. The Lynx bus system serves the city and some theme park areas but is slow. The SunRail commuter train runs north–south through downtown but doesn't reach the main tourist zones. Rideshare is widely available and often the most practical option for an evening out.