UEFI stands for Unified Extensible Firmware Interface. It does the same job as a BIOS

bios: store initialized data on the firmware
uefi: stores all data about initialization and startup in an .efi file

UEFI was designed to overcome many limitations of the old BIOS
 drive sizes upto 9 zettabytes
 faster boot up time
 UEFI runs in 32bit or 64bit mode, whereas BIOS runs in 16bit mode. 
 UEFI  is able to provide a GUI