The Undercut is a trendy haircut in which the sides and back are disconnected from the hair on top. The result is sharp angles, clean lines, and lots of volume. An Undercut cut can be swept, slicked, and styled in a variety of ways. One modern styling method is The Pompadour.
Another variant on the Undercut this style, the sides of the head are cut even closer to the skin.
An undercut can be as dramatic or subtle as you like, so long as the underlying principle remains: Long on top. Short on the sides. No taper.