OS mean Operating system. Without OS we cannot work on a computer. OS is a basic requirement for run computer. BIOS stand for Basic input / output system. It is a microprocessor system for start computer. We can do setting In BIOS. That setting apply on motherboard and system work on that setting. like, boot device select, The peripheral device on/off from BIOS. We can also check attached device in BIOS, like HDD, Cdrom, Floppy drive, etc.