A fractal is a essentially a geometric (mathematical) pattern. In fact, fractals can be found all around you in your every day life in things like plants, clouds, and even the blood vessels in your body!
An H-Tree is constructed very much like a real tree (though not random by any means). It gets it's name from the fact that when the angle between the two branches is 180 degrees, an H shape is formed. Oddly enough, this pattern has been found to be useful in designing small antenna.