BIOS stands for basic input/output system. The BIOS is firmware that's built into the motherboard that is responsible to initializes the computer's hardware as the computer is being booted. After successful boot it then searches for a boot device such as an optical drive i.e. CD/DVD ROM or a storage drive i.e. Hard Disk or SSD to boot an operating system. When computer is turned on the computer performs a P.O.S.T (power-on self-test) procedure which is executed by the BIOS to tests the computer to make sure all the requirements are met and the hardware is working correctly before starting the operating system.
If the test produce a short single beep, by the internal speaker, it signifies that the test is success and the booting process resumes normally. In case of a failure then there are no beeps or multiple beeps then that which signals the user to troubleshooting hardware issues. For example if the computer generates three long beeps then that could indicate a keyboard error or if the computer generates a continuous short beep then that could indicate a problem with a RAM module. This sequence varies vendor to vendor and manufacturers documentation provide a comprehensive instructions to pin point the issue. BIOS software is stored on the BIOS chip on the motherboard. it is a non-volatile chip which means that the contents of a chip are retained even after the power is turned off. BIOS software is persistent but its settings such as Boot order, date and time settings, Fan regulations stored in CMOS chip which a volatile piece of hardware present on the motherboard. To make the CMOS setting persist power is required which is provided by a battery, called the CMOS battery. The CMOS battery is a small button cell battery which can be seen on the surface of the motherboard and it's usually located on the bottom right hand corner and it's basically the same type of battery that is typically used in wrist watches. If the battery is removed and then reattached the BIOS will reset back to its default factory erasing any user defined settings .
UEFI stands for unified extensible firmware interface now becoming standard in almost all new motherboards shipped. This newer type of BIOS i.e. UEFI has several advantages over the old BIOS and the first and most obvious just by looking at it is that it has a user-friendly graphical user interface that supports different colors and even animations whereas the old BIOS has your typical blue screen that resembles the windows blue screen of death (BSOD).
UEFI can also recognize larger storage drives up to 2 TB. It provides a pre-operating system environment which includes a networking support. On legacy hardware it provides backward compatibility. It has a built-in feature called secure boot which help stop loading any digitally unsigned or un-trusted drivers such as Rootkits.