Welcome to tensionresolved!
Basically, this community was made for stories in which Ron Weasley and Hermione Granger, after years and years of unrequited love, tension, and frustration, finally stop being stubborn arses and get together. It's also for stories in which Harry Potter and Ginny Weasley rekindle the relationship they had which ended as the sixth book, Harry Potter and the Half-Blood Prince, finished.
The stories doesn't have to necessarily take place right after the sixth book - it could happen at anytime afterwards. There is no limitation as to when the story can take place for Ron and Hermione (meaning that the fic can take place during sixth year as well), but there obviously will be a limit as to when the Harry/Ginny fic would take place. As Harry and Ginny broke up at the end of the sixth book, it would only make sense that they got back together after it, whether during Harry's seventh year or beyond, it should take place after HBP.
You can write one-shots, you can write long multi-chaptered stories, but the point of the story should be that either Ron and Hermione finally get together and/or Harry and Ginny get back together. Your story can have both couples and any other peripheral ship in your story, but the main focus must be either of the two couples aforementioned becoming romatically attached.
There's no restrictions on rating here. You can write anything from te fluffiest G-rated story to the dirtiest NC-17 fanfic. Fanart is also allowed here, but the main focus of the drawing should be one of the two couples getting together (if not both).
The format for the heading of the story and art (which should always be put under an LJ-cut), is as followed:
Word Count (if it's a story):
-- Any comments/questions/ideas/criticisms can be asked here.