When the Raspberry Pi is powered on, it expects to boot from an SD Card containing a bootloader, from where it will find directions to a suitable operating system. The operating system is normally located in a Linux partition on the SD card, but it doesn’t have to be; it could be located on a separate USB storage device.
While it is not currently possible for a Raspberry Pi to boot directly from a USB storage device, it is entirely possible to use a hybrid-boot approach; boot from SD Card, then load and run the operating system image entirely from USB storage device. The main advantages of this approach are speed and reliability, especially when working with databases and other I/O intensive processes.
This post provides instructions for downloading and installing Raspbian onto a storage device for use with Raspberry Pi. These instructions can be used for both regular-boot SD Card and hybrid-boot SD/USB storage device configurations.
In this post, the term storage device refers to your choice of SD Card or USB storage device, unless otherwise stated.