I'm assuming your talking about automated content on your website, this is called duplicate content and will have a negative impact on your site. All your content should be unique and related to your sites niche, this will help you better SERP wise.
Thanks ssandecki, repped you. So if I have a RSS feed coverted to html from another site, this will count as dupe content. Does dupe content hurt, or just doesn't count toward SERP?
It doesn't count period since the "duplicated page" will be removed from the indexes. Therefore all links will not be followed, all links will not pass "PR juice" or anything like that. As far as the search engines are concerned (and I'm speaking ESPECIALLY about Google here), the page will not exist.
If you are planning to make an auto blog by parsing the RSS feeds from other sites, so that will dup content and the site will not get much traffic from SE's
Dan and Red, I was using an RSS feed snippet from another site which was a section on my homepage. The page ranked pretty well for my keywords, but I took it off the page to see if SERP's get better or worse to see if dupe content hurts this page, or if the RSS feed actually helps (spider activity). I still am link building, content building and such, but not really changing the rate of those as compared to the last several months. It will be interesting to see what happens.
If you do it right you can still do fine. For example, I created an auto-blog called MLBNewsBlog. It's got 43,000 pages in Google and when I owned it it did pretty well. No doubt that duplicate content isn't as good in serps as unique, but if you do it right you can still do fine
It's not an autoblog, just a snippet of an RSS feed which is updated usually every 2 or 3 days. I did this as a subsection of my main page to get fresh content when I 1st started my site. I am keeping up with the site and the homepage pretty regularly, so it's probably adding no value or maybe even hurting. I'll see what happens since I removed it yesterday.
If it's just a snippet or an excerpt then, you should be fine. The only real problem is when you duplicate the content as a single page, not a small component of a much larger page.