From NEC Retro
The PC-9821 series (PC-9821シリーズ) is a range of computers developed by NEC as successors to the PC-9800 series. Marketed as PC-98 machines for the 21st century, the PC-9821 range took NEC and their proprietary computing standards until the early 2000s, when they were finally forced to concede to the then-dominating market of IBM PC compatibles which continue to this day. NEC would in fact enter this market with the PC-9821's successors, the PC-98NX range.
PC-9821 machines are backwards compatible with later releases in the PC-9800 series. The first PC-9821 machine debuted in November 1992 (with PC-9800 models continuing to be made until 1995), and the last appeared in 1997 (although NEC continued to manufacture PC-9821 equipment until 2003, and extended lifetime support finally ended in 2010).
The PC-9821 saw its own range of software, though for the most part the machines were used to run variants of DOS or Windows (and software associated with these operating systems). The series' architecture is similar to IBM PC compatibles (moreso than the PC-9800, which also converged on this standard during the 1990s), but there are a number of differences which made the PC-9821 range incompatible with rival products.
For information, see PC-9821 series.