Basic Input Output System
Program that "teaches" the system processor how to operate with PC basic devices such as video in text mode, hard disks and floppy disk drives.
This program is stored insed the system ROM memory, which is physically located on the system motherboard. Because of that a lot of people start calling the ROM memory of the computer as "BIOS", even though technically this is wrong.
Inside the ROM memory there are three programs stored: BIOS, POST and setup. POST (Power On Self Test) is a self-test run every time the computer is turned on (memory counting, etc). Setup is the system configuration utility and is through it that we configure the hard disk type and other several hardware-related configuration options.
In the same manner a lot of people confuses "ROM" with "BIOS", the majority of people call the setup program as "BIOS" as well, while setup and BIOS are two different programs.
In summary, BIOS is a program stored inside the PC's ROM memory (which is a chip). Setup and POST are two other programs that are stored in the same chip.
On the pictures below we see the PC's ROM memory chip, which people call "BIOS" - while in reality BIOS is one of the three programs stored inside this chip, as we explained.
Figure 1: ROM memory chip. A lot of people call it "BIOS" while BIOS is one of the three programs stored inside this chip.
Figure 2: Another example of ROM memory chip.