Digital8 v miniDV
Digital8 is a Sony format and therefore you are stuck with only Sony (and I think now a few Samsung) camcorders in this format.
DV is an industry standard format used by most companies that make camcorders, therefore you have a massive range of cams and features to choose from.
You can get more, cheaper accessories for DV cams from third party suppliers.
D8 cams are a lot bigger than an equivelent DV camcorder - D8 tapes are 8mm, DV tapes are 4mm.
D8 tapes are more expensive than miniDV tapes and not as readily available on the high street/online. But D8 cams can be used with cheaper Hi8 tapes.
Some D8 camcorders can play back old analogue Hi8/8mm tapes and output them via firewire for capture on a PC. Worth a look if you have a lot of old Hi8 tapes but the old cam is now broken. Currently
AFAIK only the top of the range (about £500) Sony D8 cam is capable of doing this.
Well thats a few differences. I'm sure there are loads more, but that should be enough for now to give you a good idea as to the differences between the formats.
Mark.