In the early days of SMS, the messages that could be sent were often too short to be meaningful. As a results characters were at a premium and ways were invented to reduce a number of commonly-used words into only the essential characters. This way later became l8r. Thus a form of phonetic alphabet evolved using letters and numbers to minimize the total number of characters being utilized and sent.

As always, the abbreviated SMS 'words' have evolved and developed over the years and the dictionary presented here gives a list of as many of the current usages as I could find.
SMS Dictionary Numeric
SMS Dictionary English
SMS Dictionary Smiley
SMS Dictionary Text Symbols
SMS Dictionary - Celebs Symbols
SMS Dictionary - Smiley Faces
SMS Dictionary Text Lingo