Pokémon is a Japanese media franchise, consisting of video games, animated series and films, a trading card game, and other related media. But Pokémon themselves are mysterious creatures/monsters, living in the Pokémon universe, and can be captured or trained to battle other monsters in a trainer's quest to become the best. Essentially, they are akin to animals, though each with different skills, moves, and even types. Some Pokémon can also evolve, and they live in all different regions in the Pokémon universe.
There are mulitple aspects of the Pokémon Franchise, including the card game, the video games, and the anime/show. I'm sure there's more, but these are the ones I am most interested and well versed in.
There's a tool called a Pokédex that trainers carry around with them to identify all the different Pokémon they encounter. Click the button to reveal a Pokémon!