An online community is a term that can be used for a variety of social groups interacting via the Internet.  Some examples are, wikis which are websites that allow the visitors themselves to easily add, remove and otherwise edit and change available content, Wikipedia being the best example. Another example would a blog or chat rooms.

It is a general rule that relevant links into your website will enhance your Page Rank thus moving you up the Search Engine Rankings. It is possible to link from Internet Forums, Blogs and Wikis to your website. However, you have to consider how open the sites you are wishing to link from are to abuse, which could result in your site being blacklisted.

You will also find that different sites have different rules about linking. For example, Internet Forums may insist on you subscribing, which may incur a fee. There may also be a minimum number of postings required before you can provide a link to your site. On the other hand, you could find blogs where you can post comments without having to subscribe to anything but will not allow you to leave any anchor text link in your comment. Also, some sites may not allow links from commercial sites.  All those factors will change from one place to another so keep looking and you will find what you want, as there are millions of forums, blogs and wikis that are dealing with millions of different topics.

Adding a blog to your website has a significant effect on your site optimisation due to the new content which will attract the search engine spiders.

Posting on other blogs can provide a link to your own site but be careful who you link to. Make sure the site is of high quality and relevant to your site or you could be in danger of blacklisting.

In conclusion, online communities can provide new opportunities in optimising your site but you have to exercise the same level of discernment as you would with conventional link building. Actually an SEO company will tend more and more to use this kind of SEO technique.