Also known as a WC, a Writer Character is a character that reflects some aspect of their writer, and usually stands in for them in the story; essentially, an author avatar.
The first WCs were, essentially, the Writers themselves, posting under a certain name. As the LNH grew and became more story-based, characters separated from writers, but most writers still had at least one character modeled after themselves, and some had many (for instance, both Ultimate Ninja and Spelling Boy represented aspects of wReam; see alt.version).
This became less and less common with writers that came in after the first couple of years. Nowadays, the term is rarely used, as the majority of characters are Non-Writer Characters, or NWCs.