This is for all you people who have only seen the English dub of Digimon. No, you do not have to memorize this, as the show will mostly deal with English dub things, and if we don't we'll explain the part of the Japanese dub we will talk about beforehand so everyone understands. All this list has is name differences, basically, which is really the only thing that you would have to worry about if you haven't seen the Japanese dub. ^^

It shall be noted Savers isn't on the list of name changes here since, though a dub has "sort of" been announced it hasn't played on TV yet, and the only change I've noticed is Tohma becoming Thomas. Besides, chances are if you're looking at this you don't watch Savers so it won't matter. xD

Season One: Digimon Adventure

Most of the Digimon names stay the same...
Taichi "Tai" Kamiya becomes Taichi Yagami.
Yamato "Matt" Ishida becomes Yamato Ishida.
Sora Takenouchi stays the same, but Biyomon becomes Piyomon.
Koushiro "Izzy" Izumi becomes Koushirou Izumi.
Mimi Tachikawa and Palmon stay the same. Woo~
Joe Kido becomes... Jyou Kido! (That was amazing... It's said the same way, by the way)
Takeru "T.K." Takaishi becomes Takeru Takaishi.
Hikari "Kari" Kamiya becomes Hikari Yagami and she has a Tailmon, not a Gatomon.

The villans have the same names except Myotismon is now Vamdemon (and VenomMyotismon is now VenomVamdemon), Puppetmon is Pinocchimon, and Piximon is Piccolomon. Also in the Japanese version, Jyou has the Crest of Sincerity (instead of Reliability) and Mimi got the Crest of Purity (instead of Sincerity).

Season Two: Digimon Adventure 02 (or Zero Two)

Davis Motomiya becomes Daisuke Motomiya and he has a V-mon instead of a Veemon (OMG, big difference). Also, Demiveemon becomes Chibimon, ExVeemon becomes XV-mon, Flamedramon becomes Fladramon, and Raidramon becomes Lighdramon.
Ken Ichijouji stays the same, as does Wormmon. Woo~
Yolei Inoue becomes Miyako Inoue.
Cody Hida becomes Iori Hida, and Armadillomon is just Armadimon.

I haven't watched much of Zero Two (at least, in the proper order), which makes me a horrible person, I know. Therefore I don't really know the villans in this season. However, Digimon Emperor is the Digimon Kaiser in the Japanese version. =D I think they're all the same though... Like Arukenimon is still Arukenimon... the only one I'm not sure on is the villan at the very very end... some form of Myotismon? See, I don't even know the dub name... <<

Season Three: Digimon Tamers

Takato Matsuki becomes Takato Matsuda and Guilmon stays the same. However, Guilmon's name is pronounced as Girumon, if it matters...
Henry Wong becomes Jianliang Lee. Obviously this is a Chinese name. ^^ In Japanese it is written as Jenrya Rii, and he goes by the nickname of Jen. I have no clue how to pronounce Chinese, so I could totally butcher Henry's Japanese name by only going on the Japanese pronounciation... good thing I like the dub name better. >D
Rika Nonaka becomes Ruki Makino.
Jeri Katou becomes Juri Katou.
Kazu Shioda becomes Hirokazu Shiota.
Kenta Kitagawa AND his Digimon MarineAngemon stay the same. Weird. However, in the Japanese version MarineAngemon did talk, but she only spoke in P-sounds: pa pi pu pe po
Ryou Akiyama and Cyberdramon stay the same since they kick ass that way. =D
Suzie Wong becomes Shaochung Lee, another Chinese name, which is in Japanese Shuichon Rii.

The villans of Tamers stay the same as far as I know, though I haven't watched much of this season in Japanese. Calumon is Kurumon/Culumon in the Japanese version though, and Impmon's evolved form of Beelzemon is called Beelzebumon.

Season Four: Digimon Frontier

Takuya Kanbara is still Takuya, as it should be. ^^
Kouji Minamoto is the same, but Lobomon changes to Wolfmon.
Kouichi Kimura is also the same, and so are both his spirits!
Zoe Orimoto's name is actually Izumi Orimoto, and Kazemon is Fairymon.
Junpei "J.P." Shibayama, in the Japanese version, doesn't have the J.P. part in his name xP Therefore he's just Junpei Shibayama! And Beetlemon's Japanese name is Blitzmon.
Tommy Himi is actually Tomoki Himi, and Kumamon's Japanese name is Chakkumon. O.o

...As far as I know the villans have the same names, but some people who have only seen the Japanese dub make very creative spellings of villans (notably Cherubimon). They still have, basically, the same names though.