To clarify a little, JLKBUI;
The CMOS (Complementary Metal Oxide Semiconductor) is essentially RAM. Usually 64 bytes of memory containing system information, such as the clock, Disk, RAM and PCI bus configuration details, etc. It is battery backed to preserve settings when the computer is switched off. Many motherboards provide a jumper to reset CMOS configuration to factory defaults. This is useful if the system cannot be accessed despite a successful POST (See below).
The BIOS (Basic Input/Output System) is a program, stored on an EEPROM (Electrically Erasable Programmable Read-Only Memory). This program allows you and the CPU to access the information stored in the CMOS and modify it. If the BIOS is missing a feature that impedes proper functioning of a hardware device attached to the system, then sometimes manufacturers will release updates to the program, called flash upgrades. This must usually be done from DOS, not a Command Prompt Window, and involves a proprietary flash executable (e.g. flash.exe), along with a binary containing the new code (e.g. abc123.bin).
However, the BIOS can only be accessed after a successful POST (Power-On Self-Test). If that test fails, e.g. because of a corrupted or incorrect flash upgrade to the BIOS, then the entire system can be rendered inoperable, unless the BIOS chip is replaced with one that is compatible with the motherboard. This is because a failed POST may not allow access to Input/Output devices such as the disks.
I know this is more or less what you said, but I hope that it's clear and accurate.