A short definition? My dear sir, that would be equivalent to pouring the sands of the Sahara into a half-filled thimble. It would be comparable to funneling the waters of the Pacific into a broken wineglass. It would be like placing the collected works of Dave Van Domelen into the onboard memory of an Atari 800...however, this is the task you have set for us, and therefore I shall endeavor to elucidate.
The LNH, or Legion of Net.Heroes, is a society of those beings who emulate the spirit of adventure and undying quest for justice while clothed in spandex and a never-ending stream of bad jokes. In short, we are super-heroes, or at the very least authors who spend what spare time we have writing about super-heroes. Our stories are serious (Legion of Occult Heroes), comic (Writers Block Woman and Mouse), tragicomic (Decibel Dude and Vigilante Guy), lyrical (Tales of the LNH), or simply strange (Surreal Tales). They all take place in the same universe (except for wReam's, since he lives in his own), and authors and characters often interact with each other.
Anyone can join, although new authors can expect a bit of heckling in direct proportion to the number of times they mention the X-Men. Guidelines for writing can be found in the FAQ and then conveniently ignored. Your best policy is to read some of the stories for a number of weeks before writing your own. Most authors are willing to respond to e-mail questions about their stories, and many will even let you use their characters in stories of your own. Only wReam, however, has the power to validate your parking.
So welcome to the LNH. I hope this isn't the last we hear from you. Good luck, and get reading.