There are actually a couple of answers to this question.

One form of "technical writing," is writing that concerns technical subjects and technical fields (professions) but is written for an audience who may not be educated in those fields.  It is writing in any field of technology (from TVs, to computers, to aerospace engineering) for any audience.

Another way of explaining technical writing as compared to general writing however, could simply be writing at a level that is more "professional" in tone and style.  While general writing and technical writing both adhere to the same rules of grammar, spelling, and punctuation, technical writing tends to be more dry, less conversational (if at all), and as factual as possible.  There isn't room for opinion in technical writing.  Professional communication is not narrative writing, which includes more than just stories - but articles, editorials, letters, etc.  These have a place in the...