Operating sytem booting from CD / DVD is common these days, there is another ways of booting also there like hard disk and flash drive,
# Flash drive? it is possible?
yes it is possible to boot the operating system by using the flash drives , we will show the tutorials for booting the Ubuntu Linux operating system here
Creating Bootable Flash Drive in Windows
# Inser the USB stick atleast 2 GB
# Download the software Universal USB Installer
# After downoad Run the file when the download is completed
# If there is any Security Dialogues appears Confirm it by clicking RUN
# READ and confirm the aggrement by clicking “I AGREE”
# Select Ubuntu Desktop Edition from the drop down list
Note: we are demonstarting by using ubuntu os , the same procedure for other OS also
# Browse and Open the ISO FILE present in the hard disk
# choosing the USB Stic which is F:\ like that
CREATING A BOOTABLE FLASH DRIVE IN MAC
We would encourage Mac users to download Ubuntu Desktop Edition by burning a CD for the time being. But if you would prefer to use a USB, please follow the instructions below.
Note: this procedure requires an .img file that you will be required to create from the .iso file you download.
TIP: Drag and Drop a file from Finder to Terminal to ‘paste’ the full path without typing and risking type errors.
Download the desired file
Open the Terminal (in /Applications/Utilities/ or query Terminal in Spotlight)
Convert the .iso file to .img using the convert option of hdiutil (e.g., hdiutil convert -format UDRW -o ~/path/to/target.img ~/path/to/ubuntu.iso)
Note: OS X tends to put the .dmg ending on the output file automatically.
Run diskutil list to get the current list of devices
Insert your flash media
Run diskutil list again and determine the device node assigned to your flash media (e.g. /dev/disk2)
Run diskutil unmountDisk /dev/diskN (replace N with the disk number from the last command; in the previous example, N would be 2)
Execute sudo dd if=/path/to/downloaded.img of=/dev/rdiskN bs=1m (replace /path/to/downloaded.img with the path where the image file is located; for example, ./ubuntu.img or ./ubuntu.dmg).
Using /dev/rdisk instead of /dev/disk may be faster.
If you see the error dd: Invalid number ’1m’, you are using GNU dd. Use the same command but replace bs=1m with bs=1M.
If you see the error dd: /dev/diskN: Resource busy, make sure the disk is not in use. Start the ‘Disk Utility.app’ and unmount (don’t eject) the drive.
Run diskutil eject /dev/diskN and remove your flash media when the command completes
Restart your Mac and press alt while the Mac is restarting to choose the USB-Stick