Disneyland is the original — the template that every other theme park in the world has either copied or defined itself against. Walt Disney opened it on 160 acres of former orange grove in Anaheim in 1955, and it has operated every day since (excepting the pandemic). The park is organized into themed lands radiating from the central hub: Main Street USA, Adventureland, Frontierland, Fantasyland, and Tomorrowland are the original five, with New Orleans Square and others added over the decades.
Adjacent Disney's California Adventure occupies a roughly equivalent footprint to the south, built on what was the original parking lot and opened in 2001. California Adventure has developed into a full-day park in its own right, strongest on thrill rides and with the immersive Cars Land and Avengers Campus among its best areas. The two parks share the Downtown Disney promenade — an outdoor mall of restaurants, shops, and entertainment — which is free to enter.
A single day is not enough for both parks. Most visitors buy a two-day ticket and split their time, prioritizing the classic Disneyland rides (Pirates of the Caribbean, Haunted Mansion, Matterhorn, Space Mountain) on the first day and California Adventure on the second. The Genie+ ride reservation system can reduce wait times significantly on busy days — worth purchasing unless you are visiting in the off-season. The parks are least crowded on weekdays in January and February.