Rufus is a utility that helps format and create bootable USB flash drives, such as USB keys / pendrives, memory sticks, etc.

It can be especially useful for cases where:
you need to create USB installation media from bootable ISOs (Windows, Linux, UEFI, etc.)
you need to work on a system that doesn't have an OS installed
you need to flash a BIOS or other firmware from DOS
you want to run a low-level utility