Tag (graffiti)
Tags are one of the primary forms of graffiti, along with throw ups and pieces. The act of writing a tag is known as tagging. Tags are often thought of as the simplest form of graffiti art, prioritising legibility and flow and are usually the form of graffiti that most artists start with. Tags are, perhaps due to their simplicity, more likely to be considered vandalism than other more elaborate graffiti styles. Form 
Often done in spray paint or markers, tags are established from throw-up and pieces by being 2D, often smaller in size, and thinner lines which are the result of a speed necessity due to the often illegal nature of tagging. This necessity of speed has led to tags which are written in a single stroke called one-liners. While throw-ups and pieces may be formed from any word or even sentence, a tag functions similarly to a signature, as they are the graffiti artist's pseudonym (although rarely a personal name may be used) written in the unique style of the individual artist so that two artists with the same name would be distinguishable from one another, although an artist using a name of an existing artist in their locale or a "king" (well-respected artists) is a faux pas. An individual's unique style is called a handstyle. Tags may be existing words or made-up by the writer, but are usually only 3-5 letters or digits long, again due to the necessity of speed. Many tags use Latin script even in countries where this is not the primary writing system. About tagging, graffiti artist Kaves said “with graffiti, most people can pass by and not even give a shit about it while other people are looking at the style, the letters—the technical stuff, y’know? The many layers of it. So, learning that over the years, you start to find your own style—your own voice". History 
Tags were the first form of modern graffiti. While people have been writing their names on things since ancient times, tags were unique as stylised monikers writers used to "get up" (make their name seen by as many people as possible). The first tags originated in Philadephia with writer Cornbread, and became hugely popular in New York City in the 1960s and 70s with artists such as TAKI 183 and Julio 204 who unlike Cornbread, wrote over the whole city rather than just their own small locale. Function 

The purpose of tags is for an artist to have their tags recognised by other artists in their locale. The most prevalent taggers in an area are known as "all city", a term that originated in traditional New York graffiti. While tags are often written onto objects directly, they are also sometimes written onto stickers (known as "slaps") and stuck onto things, which is faster and safer when illegally tagging. Postal and "my name is..." stickers are commonly used for this purpose. Crew tags 
Crew tags function differently to personal tags, as they are written by multiple members of the same crew, who all have their own handstyles. Crew tags are often initials and tend to be shorter than personal tags using only 2-3 letters. They are sometimes postcodes. Crew tags are often put up next to personal tags, or used to sign larger pieces, especially if made by multiple members of a crew. References 

Graffiti and unauthorised signage
Street art
Public art