A LiveJournal community is a journal in which many users can post entries about a similar topic. Users who are interested in a particular subject can find or create a community based on this subject. For example, residents of a town or city can gather to exchange information about it or to announce local events. For an example of an actual community, see The Quest for Communities at http://www.livejournal.com/userinfo.bml?user=community_quest.
There are several differences between a community journal and a personal journal:
A community has at least one "maintainer." A maintainer is a user whose personal journal account is separate from the account of the community that they maintain. A maintainer supervises the happenings in a community, controls its settings and Community Info, and performs other administrative functions. It is possible for a community to have several maintainers who all have this administrative access.
The entries and comments in a community journal are made by other accounts that are personal journals. A community account cannot post entries or comments.
Entries made in a community journal will be associated with the user picture of the user who posted them. The community's user picture will only be shown if the entry's author has no default user picture defined and has not specified a picture to use. Alternatively, you can choose to have all communities on your Friends page show the community's default user picture instead of the author's user picture. To do so, select the option at the Modify Journal page (http://www.livejournal.com/modify.bml) that's labeled, "Use shared journal pictures instead of poster's user picture". It is not possible to select from the community's user pictures when posting entries.
In addition to http://www.livejournal.com/users/communityname/ and http://www.livejournal.com/~communityname/, a community journal can be accessed at http://www.livejournal.com/community/communityname/. If the community has a Paid Account, it can also be accessed at http://communityname.livejournal.com/.
Users can become members of a community by joining it (http://www.livejournal.com/support/faqbrowse.bml?faqid=101). It is up to the maintainer of a community to decide whether to allow everybody to join or to add members manually (http://www.livejournal.com/support/faqbrowse.bml?faqid=79).
Joining a community is different from adding it to your Friends list. Adding a community to your Friends list is called "watching" a community. You can find more information about this at http://www.livejournal.com/support/faqbrowse.bml?faqid=179.
Members are to a community as Friends are to a personal account. "Friends-only" entries in a community are therefore "Members-only"; they are only visible to members of the community (http://www.livejournal.com/support/faqbrowse.bml?faqid=83). It is not possible to create entries with Private or Custom security in a community.
You can learn how to create a community at http://www.livejournal.com/support/faqbrowse.bml?faqid=78 and how to post in a community at http://www.livejournal.com/support/faqbrowse.bml?faqid=76.