All of these definitions came from a Google search:
Straight pipe - (Straight Pipes) If the exhaust system has no baffles in the muffler the exhaust is unrestricted and flows straight...
X-pipe - An X-Pipe is, you guessed it, shaped like an x, and allows the exhaust to flow down the path of least resistance. At lower revs, turbulence is formed as exhaust gases try to shuffle past a second column of gases coming from the other side. While always producing more power than the restrictive factory system, the potential gains are not seen until higher rpms are reached. This is where an X-Pipe stands apart from the rest.
H-pipe - An H-Pipe is also shaped like its namesake and relies on exhaust expansion to balance the cylinder banks. A small section of tubing in between the main pipes provides an area for gases to expand into during exhaust pulses. Only a small amount of exhaust flows from one stream to the other as both sides push back and forth in the center section.
While both X and H pipes serve the same purpose, they use different methods to build power and economy, all while providing a distinctive sound. Think of an X-Pipe as being at home in a high-winding modern V6 or V8 while an H-Pipe conjures memories of tire-shredding torque in classic American iron.
And loud is not nessesarily better.
It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.