Tab or tablature is a system of reading music for stringed instruments. All guitarists are familiar and capable of reading tab, while for most guitarists, this is the only way they know how to read music. There are many reasons why tab is used over standard music notation by many guitarists. Its fast to learn and easy to read, and is a simple way of communicating which notes should be played on the guitar.
Introduction To Tabs - How to read the most basic tabs
Tab Symbols - Different tab symbols and what to do with them