In Web programming languages, it is the code that describes a command or instruction so that a Web browser will be able to interpret and display it. In order to link an image or word on a Web page, you must put specific tags around the image or word in the code. This is known as basic HTML and it's pretty easy. Look up the HTML definition for more info.
The word "tag" has evolved to describe a way of organizing information when "social tagging."
NetLingo Classification: Net Programming