What person is there, who is old enough to speak, who has not wondered if, somewhere out in the cosmos, in the heart of the Earth, in the depths of past aeons, or the far reaches of the future, there are other worlds? All the faiths of this Earth that I can think of seem to have them, and even those who say they do not believe in the supernatural often search for life on other planets. Fantasy and science fiction seem to be preoccupied with other worlds nowadays, whether it is Game of Thrones or Lord of the Rings, Star Trek or Star Wars. I have even met people on websites who are not normally into that sort of thing who want to write a fantasy story set in a made-up world of their own. The art of making up other worlds, as opposed to having them revealed to you by supernatural means or somehow finding yourself in one one day, is commonly known as worldbuilding. Many people would likely think that that is a very modern practice, but it is said that there is nothing new under the Sun for a reason, for it has been going on for quite a long time actually, as seen in this Slate article about worldbuilding by 19th-century American farmboys (see here for a massive collection of their pictures). Since that is by quite young children it is of course not very sophisticated, but one can be quite sure that they had not started making up that world because they had heard of Lord of the Rings or Star Wars.
Many worldbuilders nowadays seem to be extremely thorough in the worldbuilding, and make everything from languages (also known as conlangs, which is actually pretty traditional, although not under that name) and history to plate tectonics and climate features. Most however seem to focus on a few aspects, and fairly often it seems that people do not worldbuild any more than they need to write a sword-and-sorcery trilogy and then call it done (and often it seems like they fell short of having such a world, as they are full of “Common Tongues” that seem to just be English but with the words changed to sound more exotic, and barbarians who are just Vikings or Mongols with names to match, among other things). Alternate Earth/distant past/hypothetical future worldbuilders often end up having to do just as much work as completely-other-world worldbuilders have to, as seen in a recent interview about the languages in Too like the Lightning, which are just Earth languages like English, Latin, Spanish and German. I have heard it said that worldbuilding to write a story is not harder than doing research to write a story, but seeing as it takes a lot of creativity it seems like that would make up for any ease that is gained through the flexibility of being able to make things up. And if you frequent any places with amateur worldbuilders, you know that it takes a lot of creativity not to make something quite awful, and that often the same old tropes are used again and again (such as elves with pointy ears who live in the forest and who are beautiful and perfect as per the worldbuilder’s ideals).
All that having been said, let us now review various made-up worlds and things you can do, if you want/need to make up your own!