The NEC PC-9801 (which later became the PC-9821) was, loosely speaking, the Japanese equivalent of the IBM Personal Computer, from 1982 until the late 1990s. Like the IBM PC (and Fujitsu's competing FM Towns line), it used Intel 80x86 or equivalent CPUs, and could run versions of MS-DOS and Microsoft Windows, but was otherwise not IBM-compatible. For more details, see the other Wiki's article.
When released in the early 1980s, the PC-98 was considerably advanced for its time, featuring higher resolutions (initially up to 640x400) and better audio quality (using Yamaha FM synthesis soundboards) than Western computers of that era. The PC-98 eventually sold over 18 million units in Japan alone by 1999, making it one of the best-selling computer systems of the 20th century, rivaled only by the Commodore 64 (17 million units), Sharp X68000 (16 million units) and the IBM Personal Computer in sales. There were also PC-98 clones manufactured by Seiko Epson.
The characteristic dithered look of PC-98 games is due to the system's idiosyncratic display hardware, namely a pretty anemic Video RAM. Due to need to display Japanese text with its complex characters, the developers were forced to use high-resolution video mode, which didn't have much of a color depth — early models could show only 8 colors in 640×400 high-res mode. The same problem plagued early IBM PC's high-resolution EGA and VGA modes, which were initially not so popular with Western-developed games, but while Western text looked at least somewhat readable in low-res modes, Japanese developers were left without such respite.
The PC-98's contribution to Japanese video game history is a checkered one. Most earlier games for the system were essentially PC-88 games running on a faster 16-bit CPU (and identical video hardware). By the time the PC-98 overtook the PC-88 and MSX, the tide of the PC vs. Console battle in Japan was turning heavily in favor of consoles (including NEC's/Hudson Soft's PC Engine). A few important PC-oriented companies, including Compile, Falcom, and Koei, maintained support of the PC-98, though even they started developing many of their important titles for consoles. What ended up thriving on the PC-98 was not so much action games (though indie developers made some remarkable efforts) as RPGs (mainly native as well as Western), Turn-Based Strategy games, Visual Novels, and (overlapping with the previous categories) Eroge. In fact, the PC-98's game library most likely contains a greater proportion of H-games than that of any other platform (beware of NSFW images likely to turn up in searches). By the 1990s, the PC-98 had a very large library of video games, with thousands of titles, though only a small fraction of these games were ever released outside of Japan.
The PC-98's main computer rivals in Japan were the Sharp X68000 and FM Towns, released in 1987 and 1989, respectively. Despite the X68000 and FM Towns both being considerably more powerful (almost Arcade Game quality, in fact) than the PC-98, neither computer was able to dethrone the PC-98 as the Japanese market leader (but the X68000 did come close). It wasn't until the late 1990s that the IBM PC finally ended the PC-98's reign over the Japanese PC market.
Games that originally appeared on the PC-98 include:
- 4.6 Billion Year Story: The Theory of Evolution
- A-Train II, III, IV
- Cobra Mission
- Corpse Party
- Dead Of The Brain
- Dragon Knight series
- Dragon Slayer series
- Dungeon Master II: Skullkeep
- Edge (1993 Video Game)
- Enchanter: Wakaki Madōshi no Shirén
- EVE Burst Error
- Farland Story
- Giten Megami Tensei
- Legend of Heroes series
- Madou Monogatari A-R-S
- Popful Mail
- Power Dolls
- Princess Maker
- Princess Maker 2
- Raid Wind (from the creator who went on to form SITER SKAIN and produce The Tale of ALLTYNEX series)
- Red Storm Rising
- Space Pirate Captain Harlock 
- Star Cruiser
- Sword Dancer
- Touhou (the first 5 games)
- Totsugeki! Mix (a spiritual successor to Rusty)
- Uncharted Waters: New Horizons
- Variable Geo
- Wrestle Angels
- Ys series