A "blog" is a contraction for the longer term "web log" and is a word that refers to a sort of online diary for an individual person or an organization. A blog normally has a specific focus and includes posts about specific types of events or issues. A blog community is a group of people who blog about similar things or use the same blogging platform. The collective blog community of all of the blogs on the web is often referred to as the "blogosphere".
A blog community may be connected in a number of ways. A group of blogs may simply be connected because they use the same blog-building software. It is also very common for a blog community to surround a specific topic. There are hundreds of blogs, for example, dedicated to poetry. The people who run these blogs are publishers of poetry, scholars of literature who focus their studies on poetry, and poets themselves. As all of these blogs have a similar theme, they would be considered to exist as part of the poetry blog community.
Sometimes a blog will have more than one focus. There might be a blog for a publisher that specializes in printing both poetry titles and short story collections. Such a publisher would write posts relating to both poetry and short fiction. In this sense, the blog would belong to both the poetry blog community and the short story blog community. It is quite common for blogs to fall into a number of categories in terms of its content and the topics that it covers. Therefore, it also falls into a number of communities.
There are some directories that categorize the blogs on the web. It is often these sorts of directories and search engines that cluster together communities of blogs in their registers. This sort of official grouping of communities can be useful for people at the outset of a research project.
A more organic way of finding out blogs that have clustered themselves into a community is to look at the links section on a blog. The links section, which is sometimes referred to as a "blog roll," will show links to other blogs that the owner of the blog likes or regularly visits. This gives a clearer sense of which blogs have become connected to one another in their own blogosphere community.