A variety of sites exist online to publish fiction, depending on whether it is original fiction, fanfiction or collaborative roleplaying. When writing and publishing, always remember that no information posted to the Internet is safe from theft, even with copyrights.
Although anything someone posts to the Internet is automatically under copyright protection, should infringement occur, it can be difficult to prove that the author is indeed the original author. Therefore, it's best to exercise due caution when uploading.
Original fiction thrives on Fictionpress.net. Here, users can create accounts and upload original fiction for other users to read and critique. Scribophile.com is another site devoted to not just posting fiction, but getting writing tips and other help.
Fanfiction, or fiction using already existing characters and settings, is most commonly posted on Fanfiction.net. The site is subdivided into media such as literature, video games, TV shows and movies. For each work, there is a list of subdivisions for fanfiction focusing on particular characters.
Collaborative roleplaying, also known as play-by-post, has several sites devoted to it. The largest by far is Gaia Online. Gaia not only allows members to write their own collaborative roleplays, but it also allows them to create an avatar, a home, and almost an entire community within the website.