A community is journal that, most often, allows more than one person with a shared interest in something to post in a central location. Those posts can then show up on members friends list without the member having to friend each and ever other person who shares that interest. It also allows the member to post in one place that may be seen by the five hundred even if they don't all have the member's personal journal friended.

Example: A community dedicated to discussing and sharing topics related to a specific television show might have over five hundred members. Instead of friending all five hundred and having each and every post those five hundred make show on your friends list, you can simply friend the community. This way you can read posts by other members that only pertain to your mutual interest instead of reading each and every post those five hundred people made in their personal journals which may or may not be on topic.