Wildstyle is a complicated and intricate form of graffiti. Due to its complexity, it is often very hard to read by people who are not familiar with it. Usually, this form of graffiti incorporates interwoven and overlapping letters and shapes. It may include arrows, spikes, and other decorative elements depending on the technique used. The numerous layers and shapes make this style extremely difficult to produce homogeneously, which is why developing an original style in this field is seen as one of the greatest artistic challenges to a graffiti writer. Wildstyle pieces are also known as "burners", meaning "hot" as fire. Wildstyles are seen as one of the most complicated and difficult tags and are often used to get an artist's work seen (rather than to put a political message or any other kind of message across).
More information, and links, on the English Wikipedia page [1]: there is more information on some of the other language Wikipedia pages.
See also Asemic writing