Teach explained pretty thoroughly, so I don't have much to add. I do tend to post personal stuff to my journal sometimes, so I now have an account at Dreamwidth (same username) that I will be posting from. I'll still keep my LJ and have everything set to cross-post from Dreamwidth, there will just be some entries that you won't be able to comment on from LJ.
The codes that madder_rose gave are great for removing the 'repost to' choice on comments. But the only way to remove the 'Share This!' option from an entry is to friends-lock it.
Frankly, I don't care if someone reposts my fanfic entries, but I'd rather not take the chance with personal information.