Regardless of which part of the world you live in, it's almost guaranteed at one point or another you've encountered graffiti while walking throughout your town. Perhaps you love it, perhaps you hate it and maybe you've even done a little graffiti yourself. Some people consider it art while others consider it vandalism. Those involved in the graffiti scene understand it to be more than just senseless vandalism.
The graffiti characters, tags and memorials that get painted everywhere are an expression of oneself and an art. Not all graffiti is illegal, therefore it could be said not all graffiti is art. Thankfully, many councils have set up areas that are deemed graffiti friendly where artists can come and spray paint to their hearts content. This has given some artists such as Banksy notoriety outside the scene and made some artists very rich.
There are many different types of graffiti. As mentioned earlier, some people create graffiti characters to paint onto trains, tunnels and highways that run across the country. Tags or tagging is a very popular form of graffiti and it is often the name of the graffiti artist. Tags are used to show "I was here" or gangs have been known to use them to mark their territory.
With the growth of the hip hop movement, memorials dedicated to friends who have passed on have sprung up. One of the more popular memorial pieces of art is the Biggie Smalls portrait in Brooklyn. Graffiti memorials are not just for superstars though as there are many memorials painted around the world for those involved in the graffiti scene who would be unknown outside of the niche. There's usually a good story behind each piece of work so don't be quick to misjudge them as vandalism.
While some graffiti is placed on walls where it is illegal to paint such things, it's considered highly offensive if another artist paints of another persons graffiti. This has lead to competition amongst graffiti groups to cover up as much of their competitors art as possible. Often this leads to mass attacks of graffiti where individuals or parties will go out and spray their own tags and "bomb" other peoples graffiti.