Despite how it might seem from the outside looking in, there are unwritten rules of the blogosphere related to blogging ethics and etiquette. At the same time, there are legal requirements that bloggers must adhere to. Citing your sources is one of the most important rules of the blogosphere that has a legal component related to it as well.
Copyrights protect the original author, photographer, videographer, etc. from having their original work stolen or misused. In essence, if you did not create a piece of content on your blog, someone else holds the copyright to it. Therefore, you must cite your source for that piece of information (to avoid being accused of plagiarism) and seek written permission from the original owner of the content you want to republish on your blog.
Fair UseThe gray area of copyright law comes in the form of fair use. If you're reusing someone else's material on your blog for the purpose of providing criticism, comment, news reporting, teaching or research, then you may not have to seek permission. Unfortunately, fair use is debatable, so if you're republishing someone else's work on your blog, do the safe (and right) thing and ask for permission first then cite the source in your blog post.
The Importance of Linking
One of the most important parts of the blogosphere is its existence as part of the social web wherein creating communities and engaging conversations is a top priority. With that in mind, most blog authors will mention another blogger's content with an appropriate citation of the source and a link back to that source blog post (typically with a trackback). Providing attribution and a link back to the source is often enough to satisfy another blogger as long as you don't copy their content at length verbatim.
The Role of Blog Etiquette
If you cite and link to another blogger's work in one of your blog posts, be certain you add to the information provided by your source rather than simply copying it.