From Wikipedia, the free encyclopedia
A live USB is a USB flash drive or a USB external hard disk drive containing a full operating system which can be booted. Live USBs are closely related to live CDs, but sometimes have the ability to save settings and permanently install software packages back onto the USB device. Like live CDs, live USBs can be used in embedded systems for system administration, data recovery, or the testing of operating system distributions without committing to a permanent installation on the local hard disk drive. Many operating systems including Microsoft Windows XP Embedded and many of the Linux distributions and BSDs can also be used from a USB flash drive.
Proposed by IBM in 2004, in the papers "Reincarnating PCs with Portable SoulPads" (PDF & Summary) and Boot Linux from a FireWire device. 
Benefits and limitations
Live USBs share many of the benefits and limitations of live CDs.
- In contrast to live CDs, the data contained on the booting device can be changed and additional data stored on the same device. This allows for live USBs to be used as personal storage, as it allows a user to carry their preferred operating system, applications, and configuration as well as personal files with them, making it easy to share a single system between multiple users.
- Live USBs provide the additional benefit of enhanced privacy, because the user can easily carry the USB device with them or store it in a secure location (e.g. a safe), reducing the opportunities for others to access their data. On the other hand, usually it is easy for a USB device to become lost or stolen, so data encryption and backup is even more important than with a typical desktop system.
- The absence of moving parts in USB flash devices allows for faster seek time than is possible with hard drives or optical media, meaning small programs will start faster from a USB flash drive than from a local hard disk or live CD. However, as USB devices typically achieve lower data transfer rates than internal hard drives, booting from a computer lacking USB 2.0 support can be very slow.
- Some computers, particularly older ones, may not have a BIOS that supports USB booting. Many which do support USB booting may still be unable to boot the device in question. In these cases a computer can often be "redirected" to boot from a USB device through use of an initial bootable CD or floppy disk. 
- Due to the additional write cycles that occur on a full-blown installation, the life of the flash drive may be slightly reduced. This doesn't apply to systems particularly designed for live systems which keep all changes in RAM until the user logs off.
Principle of installation
Various applications exist to create live USBs; examples include the Fedora Live USB Creator and UNetbootin, which works with a variety of distributions. A few Linux distribution and live CDs have ready-made scripts which perform the steps below automatically. In addition on some, extra applications can be installed, and a persistent file system can be used to store changes.
To install a live USB system on a memory stick the following steps need to be done :
- A USB flash drive needs to be connected to the system, and be detected by it
- One or more partitions may need to be created on the USB flash drive
- The "bootable" flag must be set on the primary partition on the USB flash drive
- A MBR must be written to the primary partition of the USB flash drive
- The partition must be formatted (most often in FAT32 format, but other systems can be used too)
- A bootloader must be installed to the partition (most often using syslinux when installing a Linux system)
- A bootloader configuration file (if used) must be written
- The necessary files of the operating system and default applications must be copied to the USB flash drive
- Language and keyboard files (if used) must be written to the USB flash drive
Types of live USB
Live CD derived
The first type of live USB is created by simply taking the ISO image file from a live CD distribution and placing it on USB storage device and then making it bootable.
- Can be simple to install with no risk to other systems on hard disks.
- The compressed format allows many applications within the limited storage available.
- Updating the system can be as easy as replacing a single file.
- An installation can be as small as 50MB.
- Many live CDs are set up to be USB-aware, and write changes to a second partition on the USB device
- Some live CDs are not set up to write to their own filesystem because a CD is typically read-only, thus it can sometimes be difficult to enable a live USB OS to store changes.
- Making the ISO image bootable, and partitioning the USB device correctly, can be difficult.
The second type of live USB is closely related to a traditional operating system hard drive install with minor modifications like the elimination of swap partitions and files.
- Updating applications or the whole thing is as easy as the parent distribution used to create it.
- Full system encryption possible.
- Easier to customize with the user's preferred Window Manager and applications.
- Base install usually starts at approximately 200MB (although some can be as little as 40MB) and grows as the user adds applications.
- Only easy to install if the operating system provides support.
- Will wear through the USB faster.
- BackTrack: A distribution of linux that is made with security in mind and is commonly used for cracking WEP passcodes and has live installs including USB and CD.
- BeleniX: Customized OpenSolaris installs including live CD and live USB.
- Billix: A multiboot distribution and system administration toolkit with the ability to install any of the included distributions.
- Damn Small Linux: Knoppix derived small installation, uses a 2.4 kernel, with JWM as default user interface.
- Devil-Linux: for service installations e.g. firewall/router, no graphical interface, can boot from CD or USB.
- dyne:bolic: 100% free distribution for multimedia production, boots from CD and USB stick, with Xfce as default user interface.
- FaunOS: a live USB distribution based on Arch Linux
- Fedora (with Fedora Live USB creator)
- Gentoo Gentoo USB Live.
- Knoppix: Full live CD/USB based on Debian distro.
- Knopperdisk: A small distribution based on Gentoo but designed to be run from USB pen drives or floppy disks.
- MCNLive: Mandriva derived live CD, and live USB distribution.
- MilaX: Small OpenSolaris live CD and live USB distribution.
- NimbleX: A small (200mb) distribution based on Slackware.
- OpenBSD 
- OpenSolaris: The Distribution Constructor project has tools allowing users to build an install image.
- OS-9 RTOS: The standard OS-9 Configuration Wizard for X86 creates bootable USB sticks. OS-9 5.0 for X86 will support the creation of bootable CDs.
- PCLinuxOS: Version 2009.1 comes with a live USB creator tool, version 2008 "MiniMe" can be installed manually
- Pentoo: Gentoo based live CD and live USB distribution focused on penetration testing and security assessment.
- Puppy Linux Designed for easy install on USB.
- RUNT Linux: Based on Slackware with a 2.4 kernel and Umsdos filesystem. Designed as a network testing tool for students at North Carolina State University.
- sidux: Based on Debian unstable (sid)
- Slax: Slackware based installation.
- SliTaz: Very small distribution.
- Sugar (desktop_environment) - Sugar on a Stick is a LiveUSB for children and learning.
- Super OS: usb-creator and cd2usb already included on the DVD
- Ubuntu (can be installed directly to a flash drive or USB external hard drive manually by using tools like usb-creator, UNetbootin, or cd2usb)
- Windows Preinstallation Environment: Freely available version of a live Windows installation, command-line only.
||Alternatives to live Cd creation
||Netinstaller (downloads iso & makes Usb), UNetbootin
||Zip + sh&bat scripts
||2ram (gobolinux toram)
||USB installer GUI
||in folder, on USB stick
||Zip + sh&bat scripts, UNetbootin
||none & from internal drive($tazusb)
||in hacker folder
||through script (Tazusb)
||2ram - lowram
||none (Control Panel)
||auto after making permanent space (Control panel)
||AllUsb - 2Ram
Syslinux is a program that makes a USB storage device bootable (they are often used after extracting files to the formatted media).