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:

1) you need to create USB installation media from bootable ISOs (Windows, Linux, UEFI, etc.)
2) you need to work on a system that doesn't have an OS installed
3) you need to flash a BIOS or other firmware from DOS
4) you want to run a low-level utility