BIOS = Basic Input Output System . Refers to the basic software on a CMOS Chip on the Motherboard and is used to "configure theother chips and hardware attached to Motherboard.
CMOS = COMPLIMENTARY METAL OXIDE on SILICON.Refers to a certain type of low voltage Integated Circuit chip.
DOS = DISK OPERATING SYSTEM.Pre Until Windows 2000 & NT all Windows Operating Systems were written in a DOS shell which required precise SYNTAX commands.
It is really only used in XP if type in "CMD" for Command in the RUN box from START.