An online community ist a special form of society of humans who meet in the internet and interact with each other. If this interaction happens in a social network which acts as a plattform to exchange opinions, experiences, news and advices, it's called social media. (translation from wikipedia)
Let your visitors be part of your site. Let them help you keeping your site up to date an keeping your website attractive. In addition they help you to feed the search engines with stuff to index.
Community marketing is the most advanced form of online marketing.