Is this illegal to copy from wikipedia and use in your articles and blogs? Do they also have a copyright on all their entries? They are open source though
No, you don't cut and paste the whole article. Quoting a sentence or paragraph and linking to the source is fine.
Actually I have the understanding that it is OK to copy an entire article from Wikipedia. I have never done it personally but I believe that is the case.
From the wikipedia faq Can I reuse Wikipedia's content somewhere else? Wikipedia's textual content is copyrighted, but you may reuse it under the terms of our licensing requirements, summarized below. Text in Wikipedia, excluding quotations, has been released under the GNU Free Documentation License (or is in the public domain), and can therefore be reused only if you release any derived work under the GFDL. This requires that, among other things, you attribute the authors and allow others to freely copy your work. (This is a summary, see the licence text for the exact details.) If you are unwilling or unable to use the GFDL for your work, use of Wikipedia content is unauthorized. Small quotations of Wikipedia content, with its source attributed, may be permissible under the "fair use" clause of U.S. copyright law. See Wikipedia:Citing Wikipedia for information about the proper citation of articles. No permission is needed to create a hyperlink to Wikipedia or its articles. Images used in Wikipedia may have their own, completely independent licensing scheme. Looking at an image's description page by clicking on the image itself should ideally tell you the copyright status of the image. Many images are either in the public domain or licensed under copyleft licenses (such as the GFDL), but many are copyrighted and used on Wikipedia under the "fair use" clause of U.S. copyright law. en.wikipedia.org/wiki/Wikipedia:Copyright_FAQ
I've done it. You know what you do? Give credit! Then if someone actually comes after you (slim to none) you look like a good guy at least.
Pity Wikipedia dont follow their own agreements and credit the source. Ive got several documents on there written soley by me yet editors removed my links. My site links are directly related and contains further info, yet some editor sees its his duty to remove them when they were fine for 18 months. Simple, i will use content from there and link to the original authors site if available but no way will Wikipedia get a link from a domain i own ever again.
Wikipedia credits the author through the history page. Meating the terms of the GFDL without linking to wikipedia is doable but a lot of work.