Yes, I figured out that it was about time I had my say in the Pokémon vs. Digimon debate, and try to clear out a few things. Well, as far as I can get it anyway, which isn't much since I'm neither Japanese or totally obsessed. Anyway, this is the story as I know it (yes, something is most probably wrong since my sources were all from the internet, a thing you should not place too much trust in, grasshopper):
Digimon had its mer start in the small Japanese... um... things, you know, those that bleeped when it disturbed the most and were all over the world for a year then to disappear never to be seen again. Yes, the Tamagotchis.
Digimon started as a sort of advanced Tamagotchi, were you could fight with others. Or something. Whatever. Point is that Digimon started as a sort of toy, which was extremely popular in Japan, and since Japan is a commercialized country indeed (Pokémon, Digimon, Sailor Moon, Mario Bros, you know were they went), the anime was not far behind.
It was aired in the US in 1997, but since almost nobody watched it it was taken off pretty quickly. Then the Pocket Monsters entered the Western world a year after, and cute Japanese monster fighting each others and changing shape when needed was a thing you could make good money off. So guess which show that was once again released, this time with great success?
Such is the story of Digimon, and it states that Digimon is NOT a Pokémon wannabe (but of course, it did take advance of Pokémon's popularity when released in the US, which hardly can be blamed on the show itself.)
But.. they are similar!
At the first look it might seem so, yes.
They have almost similar name with almost similar meanings (Pocket monsters/Digital monsters), they both have a lot of different creatures (350 Pokémon, over 500 Digimon) which can change shape to get stronger, the monsters fight each other, their attacks are not random, there are kids making the monster fight and it takes place in fantasy worlds.
And yes, it is true. But there are differences in this which makes them completely different as shows. Now, follow me on my feeble attempt on explaining them:
Pokémon vs. Digimon
Pokémon is the short version of the phrase "Pocket monsters", referring to monsters you capture in a ball small enough for you to carry with you wherever you go.
Digimon is short of "Digital monsters", monsters created in a digital world and controlled by a digivice.
Monsters in balls vs. Monsters with Digivices
As you should know, Pokémon as monsters kept in balls. They live like normal animals in the wild until they are captured and trained to fight other Pokémon.
Digimon come out of eggs. They live in the Digital world but can come to the real world as well, and they are connected to the people who has their digivices.
Evolving vs. Digivolving
Pokémon evolve. That be by growing a level, evolution stones, happiness or trading, most Pokémon does evolve into a better kind of Pokémon when the time is there.
Digimon, on the other hand, digivolves. But while pokémon has one and only one evolution (except Eevee and Gloom) Digimon can digivolve different ways.
They can just digivolve normally with the digivice (there are... um... five different stages, I think), they can armor digivolve (with the help of some thing in the digivice) or DNA digivolve with another digimon. (Then there's the 3rd season where they digivolve with the digidestined, but I don't have a clue about that), and more important, they don't stay. A digimon can digivolve to its ultimate level, but will after a while return back to whatever it was before it evolved.
(Digimon fans are all allowed to boo at me, because I don't have a clue except the very little I've seen of the show itself)
The Pokémon universe vs. The digital world
The game, manga and anime of Pokémon takes place in the Pokémon world, which as far we're concerned are the continents Kanto and Johto (and the Orange Islands in the anime).
Digimon does on the other hand take place two places, the Digital world and the real world. The kids live and love in Tokyo.
The Pokémon anime vs. The Digimon anime
Yes, there are pretty big differences in the two shows.
Pokémon is a show with a very simple plot: Watch Ash Ketchum become a Pokémon master.
Digimon is a show with a very predictable plot: Save the world.
Those two things does already set the difference between them as huge. Yes, Ash and his friends and foes have saved the world in no less than two movies (I don't have a clue about the 4th yet), but the anime doesn't have any big plots except Ash's endeavoring to become a Pokémon master.
Digimon has tons of them. One thing is that the Digidestined are supposed to keep the balance between the Digital world and the real world, but they do also have their problems inward in their own groups, like how Tai and Matt kept on rivaling about the leadership in the 1st season.
Digimon does also (partially) take place in the real world. The digidestined are children living in Tokyo, in more or less normal families and having more or less normal lives (except saving the world with their digimon). This also makes Digimon face a lot of darker themes of life, like divorce, adoption and death.
In Pokémon no such things happened, except Brock's mother who died (she ran away in the Japanese version). I mean, Ash Ketchum has no father, Tracey has no past and Misty has a pretty cold relationship with her sisters, and yet that is nothing to bother with.
In Digimon, TK and Matt are brothers, but lives with a parent each. To take one thing. There are actually quite a bit more.
So they are different, and very so
And why do I bother?
Why, I like Digimon! Shame as it is I can't get it myself, but I've seen some, and I do love it, and I think the random hate for Digimon in the Pokémon fandom is wrong. Neither copies the other, and that's it. Now go do something useful instead of reading this crap written by a person who is so not the right one to explain it to you.