Q:  I just wrote a blog post that comments on what someone else wrote.  That’s okay, isn’t it?
A: We like the rule-of-thumb that says, “If you wonder whether it’s plagiarism, it probably IS”.  Another way to think about it is to ask yourself, “Did this come 100% from my head?”  If the answer is no, cite your source at a least, link to the source when you can and asking permission is always a good thing.

Q: But I can link to other people’s sites, right?
A:  Probably, but good manners (and perhaps the law) say you should ask and, if the site owner doesn’t want you to link to them, don’t.  Blogger Brad Templeton has a nice overview of this topic which is surprisingly not as straightforward as you’d think.

Q: I used someone else’s photo, but [a] it’s for a non-profit  [b] it’s for my scrapbook [c] it’s just on my facebook [d] I just emailed it to friends.  That doesn’t break the law, does it?
A.  Yes, it does.  Taking what came from someone else’s head, be it words or photos or things or ideas, is stealing, which is what copyright is out to protect.  There are degrees of theft and consequences, but stealing is stealing.

This site has a brief overview of the Digital Millennium Copyright Act and the Engine of Creation.

This Squidoo Lens is a quick read on plagiarism, copyright and fair use.