This illustration is worth at least a 1000 words.
Never heard of PC98? Lucky you. For those less fortunate, PC98 is a nightmare created by NEC (a "cool" Japanese company that makes nifty gadgets). What's wrong with PC98? Well, everything. Let's start from the beginning. The hardware.
Thankfully, NEC haven't produced much of PC98 since making their last (I hope) model using Celeron 600 processor. They seem to have jumped directly from Pentium to Celeron since I don't recall ever seeing any PC98 with a PPro or P2 chip.
It's pretty easy to spot a PC98 by just a quick look at the back. On newer models, you'll find one or 2 PC98 PCI slots to the right, immediately preceeded by 3 or 4 horizontal "backplane" slots where the NEC-proprietary extension cards plug in. These range from SCSI cards to connect to utterly useless MO drives, 14.4K modems, 10BaseT ethernet cards, and of course, 16bit soundcards (usually using AD Logic chipsets). These cards will NOT work (or even fit) into any standard PC hardware, so forget about exchanging them. Newer models include "PCI" slots. However, those are also proprietary, and standard PCI cards are unlikely to work in them. This is partly due to the fact that there are PC98 "video accelerator" cards, that contain the graphics chip and memory, but no VGA connector - the card sends video down to the motherboard where its put back to the original VGA connector. I've seen some Matrox graphics cards like that, utterly useless on a normal PC since they cannot output the video signals.
PC98 serial, parallel, keyboard, and mouse ports are not your "standard" type. They all use NEC-proprietary connectors. Want to print from a PC98? Better buy a PC98 dot-matrix printer.
This illustration is worth at least a 1000 words.