In the world of writing on the internet, there are two terms that are often used interchangeably but are actually quite different. Those two terms are “FCs” and “OCs.”
“FC” stands for “fan character.” A fan character is a character that is created for fan fiction. The character itself may be a completely new creation on the part of the fan fic writer, but the universe that this character is being placed in was NOT created by that writer. For example, in my Fruits Basket fan fic “Crane Dancer,” I have an FC named Tsuru Odoriko. She is not part of the Fruits Basket canon, or even a side character. She is entirely my own creation, but the universe and other characters she interacts with were made by Natsuki Takaya, the author of Fruits Basket, not me. Thus, Tsuru is an FC. An FC can be well-developed and able to stand proudly or it could be a blatant self-insert or overly-perfect Mary Sue. A Mary Sue is a specific kind of FC, a character that is so overly perfect or powerful that they do not feel like a real, flawed human being. I know that some of my own FCs like Lyra Whitefall Palgrave started out as Mary Sues, but I’ve been working to correct that, to make FCs like her less powerful and perfect and more like real people. Self-inserts can be over-idealized versions of the author dropped into the story. Many amateur fan fic writers do this because they lack practice creating characters. (“Mary Sue, Mary Schmue” by Birde Williams is an excellent article about the proliferation of Mary Sue characters in fan fiction.)
“OC” stands for “original character.” A lot of times FCs are incorrectly referred to as OCs since the FCs themselves are “original” within the context of the fan fic. However, there is a distinction. OCs are characters created for a world that the authors has also had a hand in creating. For example, Irene and Matthias are OCs because they were created by me for my own story Astral Rain. Any character that you create for your own worlds are your OCs. Obviously OCs can run the gambit of cardboard to three dimensional with their own mix of Mary Sues and author-self-inserts. A writer has a lot more work to do with OCs since they have to create the world as well as the characters and be sure that everything makes sense. Sometimes FCs can even become OCs. In my Harry Potter fan fic “Sundered Blood,” I have an FC named Samantha Halfward. Right now she is an FC because I created her for the Harry Potter universe. However, since I like Samantha, her background, and other creations that I made for her story so much that I’m planning on eventually giving Samantha her own universe to play in. If I do it right, Samantha will transition from a fan character to an original character.
Regardless if your character is an FC or an OC, equal care should be taken in their creation and execution throughout your story. One is not inherently better or worse than the other, although keep in mind that the tales of OCs can be published for money but the tales of FCs cannot.