One way to be part of the blogosphere is to read and comment on other blogs. Comments are a way of joining the conversation. Often, bloggers will actually prompt their readers to respond. Comments provide an outlet for readers to join in and discuss the issues posted in blog entries.
Normally, when leaving a comment it requires you to include your name, email address, and the URL of your own blog (if you have one). A link under the post will actually say “Comments.” By clicking on this link, an area will open up allowing you to post your thoughts. Some blogs permit for comments to publish right away, while others will hold the comments for the blogger’s approval. Another option is to require comments to be registered members of a network (like at Blogger.com) or there might be a special series of words or numbers that must also be entered by the person leaving the comment.
All of this is to provide a way to limit spamming. If a blogger opts for comment approval, she can go in later and either approve or deny the comment to be published. Safeguards such as registration or special words require extra steps from the person commenting, so this often will deter spammers (though not always).
Other than commenting to be part of the blogosphere, another reason for commenting is to leave links back to your own blog. This way, you automatically add external links which point back to your blog. It is also a pretty good way for blog readers to find other blogs of interest because very often those leaving comments have blogs on the same or similar topic.