Thneedville is a city in The Lorax. All the characters except for The Once-ler and The Lorax live there. It was originally named by The Once-ler based on his hit creation, the Thneed. The mayor in the movie was Aloysius O'Hare, owner of the O'Hare Air Company. It's also possible that ThneedVille changed a bit when the townsfolk ditch O'Hare and allow Ted Wiggins to plant the Truffula seed and grow it.
Until the seed was planted, there was no nature in the city, and most of it was made of either plastic or electronics.
Thneedville was designed as a utopia town by the Once-ler to further advertise his product after which the place is named (the Thneed), but just as soon as his model was becoming a reality, the last Tuffula tree fell and the Thneed factory closed down. Over time, O'Hare took over the town as mayor, being seen as a hero for selling oxygen (though he had to tighten the security to keep the citizens from finding out they can get fresh air for free just by planting a tree). Everyone (except Audrey and, for her sake, Ted) sees the place as perfect, even though everything, including the "plants", has been manufactured in a factory and bought in a store... they even let some kids swim in the over-clorinated "pond" to the point where one or two of them glow in the dark, proving the locals overlook the pollution right in front of their faces... that is, until the walls came down and everyone (except the impeached O'Hare) decided it was time for a change, and grew a Truffla tree in the park in the middle of town.