I'm not sure in what order things appeared but there are many different aspects to Pokémon and they are not all bad.

General Info
While there are differences between the game and the cartoon, here is some basic info on Pokémon in general. The main character is named Ash, and he lives in an alternate universe that I'm pretty sure is called the Pokéworld. Besides humans, this world is filled with creatures called pokémon, or pocket monsters (which is weird since neither the creatures nor the pokéballs fit in a normal person's pocket). It's Ash's goal in life to become the Ultimate Pokémon Master by collecting all 150+ pokémon and winning a tournament. The pokémon vary greatly in their appearence, size, and abilities. After a pokémon has gained enough experience they become stronger and learn new skills like in any RPG. After gaining a lot of experience they actualy evolve into a different pokémon which is bigger and stronger. Most can do this twice, and some never evolve. One collects pokémon by first beating them up a little with the ones you already own and then trying to capture them in a pokéball. The pokéball is white and red and is one the most common symbols in its advertising. If the capture works, the pokémon somehow is zapped into the ball and can be released later to fight or whatever. The goal of a pokémon trainer is to develop their pokémon to be as strong as possible and to evolve as much as possible. It's a good idea to have a good mix of different kinds of them and to know how to use their abilities well. A common practice among pokémon trainers is to either duel or have tournaments. They duel by having some of their pokémon fight one at a time against their opponents. One can usually only use like 6 pokémon in one duel. The winner is the last trainer with a pokémon standing (they never die, just faint). Oh, pokémon can't really talk. They can just use the syllables in their name. Thus the annoying "Pika Pika, Pikachu!", etc. The ability of Pikachu to communicate with Ash is almost Lassy-esque.

The Cartoon/Movie
This is your basic run of the mill kid's anime. It's almost painful for adults to have to watch. What annoys most people is how integrated the marketing is in the show. Kids are constantly bombarded with the phrase "Gotta' Catch 'em all!" The themes of the show are like any other kid's show, except sometimes focusing on concepts from the game (like the merits of forcing a Pokémon to evolve early). The cartoon features the standard idiotic villains named Team Rocket. By far one of the most annoying aspects of the show is watching these bumbling fools. For some reason Team Rocket's main pokémon, Meowth, can talk like the human characters.

The GameBoy Game
While the cartoon is horrible, the GameBoy game is actually pretty cool if you give it a try. Even my brother, who's 18 and not at all a geek, liked it after getting over the stigma of its popularity among kids. It's very similar to the game Earthbound, which is also a kid's game that is fun for adults to play for some reason. In general, it's just like a Final Fantasy type RPG. You walk around gaining experience, buying stuff, doing mini quests, and trying to accomplish an overall quest. The main difference is that your team can be made up of the same monsters that you fight. You play Ash in the game, and you have no annoying sidekicks like in the cartoon. At any time you can carry around 6 pokémon with you. When you fight you send out a pokémon to do the actually battle. If one faints then you can use another one. If they all faint you lose. Nicely avoids the death issue. There are two types of fight. Your pokémon versus one wild pokémon, or you versus another trainer. The later is more interesting because the other trainer has several pokémon too. What's really cool is that anytime you encounter a new pokémon you can add them to your team if you manage to capture them. As you go along you have to try and increase the experience of your favorite pokémon. In addition to normal level bonuses, having a pokémon evolve is pretty cool. If you've ever played Final Fantasy II you should like this. I loved in that game when your dinky little guys turn into their cooler older versions. Well, in Pokémon it happens a lot with all of the characters. Another important thing to mention is that the game is farely strategic. You actually have to think about which pokémon to use and what attacks to use. Another important thing to note is that Team Rocket is much cooler in the game. Instead of a couple of dolts, it's a weird underground Mafia-like organization trying to take over the world. I'm surprised not many more people are impressed at the new ideas this game introduced to RPGs. Anyway, on top of all that the true stroke of genius was to include a way to have real life players fight and trade their pokémon. This brings a great social aspect to the game. This is a great game for kids to play. They really have to use their brains to get very far in it, and it introduces them to the wonderful world of fantasy and RPGs.

Other Games
Pokémon Snap is actually kind of fun too. The player takes rides through various pokémon infested locations and tries to take pictures of them. It has a pretty cool system of judging the pictures.
Pokémon Stadium is rather silly. It's only real use is if you have a group who wants to fight their battles in a 3D with good effects. It's really not a stand alone game. It also includes some really boring mini games.
Pokémon Card Game: I don't know much about this. I suppose it's something like Magic: The Gathering.

Pokemon has also spawned at least three movies so far. The first one introduced Mew and Mewtwo (sp?). I've not really watched any of them so I can't really comment on them.

Digimon is not a spin off of Pokemon. In fact, Digimon may have come first. Go there for a discussion about it.