This means cute as a cat or puppy, the ‘:’ are the eyes, obviously and the ‘3’ represents the jowls of a dog or the cute fixed smile cats tend to have on their face. This symbol or expression is called an emoticon; a pictorial expression that is presented in letters and all manners of punctuation. It is used to convey an emotion from one person to another via computer or mobile phone text messaging. It is used to enable expression or add ambience to plain text on the screen.
When using a forum, for example on online gaming or social networks such as Twitter or chat rooms, the form of communication is instant messaging. It is not easy to give emotion to conversations, as there usually isn’t very much time or space to give long sprawling prose about the issues contained within, so this shortcut sprung up. It has been used since 1981 although it was used periodically and not quite in the form as we are accustomed to it now.
The creation of symbols such as ;-) are credited to Scott Fahlman (1982) who specifically used them to express an actual emotion, he used it in a suggestion to his peers at his University and it spread like wild fire. All the variations that came from this initial emoticon are symbols that can only be read at a head tilt angle. In 1986 techs from East Asia brought to life emoticons that can be seen without having to tilt your head, it was discovered in ASCII NET in Japan. The smiley interpretation strangely is not too far from the Japanese anime (_), asterisks are the eyes, the underscore is the mouth and the brackets either side signify the wide face. Another example is =), and :).