Pokémon referred to the creatures that can be obtained in the successful Pokémon franchise. Currently, there are a total of 649 types of Pokémon species, with about 150 or so new ones being introduced with each new generation, or series of the games. In the first generation, which consisted of Pokémon Red and Blue, and the counterpart Pokémon Yellow, there were a total of 151 Pokémon.
Each Pokémon is categorized into a multitude of traits, including their type, color, weight, height, and so forth. Most Pokémon can evolve into a more powerful species by leveling up, sort of like how a character in a traditional RPG can change into a more powerful level.
Pokémon in the anime, have been shown to only be able to speak their name or a variation of it, excluding a select few who can actually speak English (example - Meowth). Others, such as Mewtwo and the ever popular Lucario can speak telepathically, but not directly to a human.
Legendary Pokémon tend to be the most powerful types. They're usually the mascots of a particular game and are found near the end of the playable character's adventure where they'll be given the chance to actually capture it.