The exact origin of graffiti and tagging is unknown because it has been done for many years. In 1971 the "New York Times" ran an article on Taki 183, a man who had begun tagging all over New York city subways.
Tagging is another term for creating graffiti. People who create graffiti are called taggers. In most places tagging is illegal because it is considered vandalism. Authoritative sources do not consider graffiti to be an art or self expression, but rather a problem in communities. The main reason that tagging is considered a problem is that it is often done to public property and the city has to pay for it to be cleaned up. Also, most graffiti has specific meanings attached.
Graffiti comes in many forms but the tool needed for all forms are the same: spray paint. More detailed graffiti may require the use of stencils and extensive use of colors. When people paint large areas, this is called "bombing." Different tips can be used on spray paint cans to get the desired effect an artist wants. In addition to spray paint cans, some taggers use paint pens.
There is a whole culture surrounding graffiti, spray paint and tagging. Taggers typically wear baggy clothes because they carry spray paint cans around in case the opportunity to tag is presented. Also, there is a whole vocabulary that comes along with this. People have their own nicknames and may belong to crews that have nicknames as well.
One danger of tagging is that gangs use it to mark their territories rather than to create art. They are often just a set of letters or numbers that represent the name of the group or set.