Some models of desktop computers and laptops are not equipped with CD or DVD drives and it is recommended to use a USB flash drive to install operating system. The main question is how to write the operating system to a USB drive. Bootable USB flash drive is created using Rufus in few clicks. Official site offers versions:

Rufus is a standalone app designed to format and create a bootable USB drive for a large variety of ISOs. The app is just over 1MB in size and is notably faster than similar competitor apps such as the Windows 7 USB utility, Universal USB Installer, and UNetbootin.

Rufus Download is a free application that could use to format and create bootable USB flash drives like pendrives, USB keys etc. Especially, it can be used for some helpful situations such as make a bootable device to install Windows, Linux, Ubuntu and more from ISO, work on a system that hasn't installed an OS, flash a BIOS or any other firmware on DOS, work on a short level utility.

Rufus is a small program for converting a normal flash drive into a complete bootable USB from which you can directly start in DOS. Configuring the program is as simple as inserting the device you want to format, marking the options you want, and clicking "start." In just a few seconds your new bootable USB for DOS drive will be ready.

Now Rufus will start creating bootable USB drive for you. It will take some time depending on the ISO image size and USB drive type. Once the process completes, the program will intimate you by a sound and you will see the progress bar completely green colour.

Rufus is a freeware tool for formatting USB disks so that they may be used to install alternative operating systems to run a live distribution of Linux. It also supports the creation of a MS-DOS (or other flavor of DOS) boot disk. The menus here in Rufus are similar to those which may be found in Microsoft's own Windows formatting wizard.