Lifting someone else's posts is plagiarism.
A common practice in the blog world these days, which I believe is called "scraping", is when someone will lift another blogger's rss feed and plunk it right down on his/her site without permission, attribution or even a link back to the original site. Let me be clear here: you're profiting from someone else's content and essentially pretending that this content is yours, even if you're not explicitly writing out, "I wrote this." Consider what this would be in the non-digital world: you copy and paste the entire text of Lorrie Moore's new novel and sell it. Yes, that would be illegal. So you can see how the same practice is wrong on the internet, right?
A blog post made up entirely of other's work, even with attribution, is plagiarism.
So you put the author's name at the bottom of the text you scraped. Whoop-de-doo for you. You're profiting from another author's content without his or her permission, basically claiming ownership for the text by having it as your content on your blog. Really, it's just laziness in this case; you want to have a blog, but you don't want to expend the effort to keep it up.
After the jump: ways to stay one jump ahead of the cheaters!
Printing out a poem or story you find online, even if not by a professional author, and calling it your own, is plagiarism!
I've been a victim of this trick! Because I used to post my work on the site, a high school student in a creative writing class across the country tried to hand in my work as her own. Luckily the teacher was on the alert, found my site, and contacted me. It's amazing how blatantly some people are willing to disrespect the work of others. And while that story may not have been published and wasn't as recognizable as an Updike or a Salinger, it was still my work and no one else's.
Now I'd like to offer a few tips for avoiding this kind of annoyance.
Don't post your work in its entirety online.
I've learned over the years that it's just not wise to publish your work online, first because people will steal it, and second because future journals may be strict about getting first-time rights. You don't want to ruin your story's chance of publication by putting it on your site, so don't! If you like, post a poem or two, perhaps one story or story excerpts, but keep the web publishing of your actual creative work limited.
Search the web for thieves.
If you're worried about other bloggers stealing your posts, try a regular search of your page with Copyscape, an anti-plagiarism search engine. It'll search the web for sites that contain the same text as your page.
Know your rights.
The first line of defense in cases of plagiarism is to know the basic facts about what rights of ownership you have to your work. Check out my post on Copyright basics, which will get you up to speed. If you're armed with the facts, then you'll be better able to come down hard on a site that's stealing your work. So good luck, writers; be honest, be respectful, and be vigilant!