I have thought about start to develope a site. At the moment I need a lot of content (text). I have been wondering if I could use content from other sites. Some why I am not so sure if it is legal. Is it legal if I add link to the site where I have copied the content and I don't charge anything from the user for reading the copied content?
Could someone talk through what kind of rights do people have for content coying? How much can I copy from other sites if site have copyright label?
No, its not. Unless they site says somewhere that you may to use their content on your web site, or you contact the site owner and get their permission.
Never use someone else's work without permission. Copyright laws apply regardless of whether or not the site has a copyright notice or symbol.
It cannot be used on your site if the site you are getting content from gives you a right to use it. Or it falls under other type of licenses like Wikipedia falls or commons creative license.
Some sites also allow to copy articles if you keep the resource box intact.
Sites that allow you to use their content say so explicitly and will detail the terms of use someplace on the site (or point to a license like Creative Commons, GNU free documentation, etc). Some examples are Wikipedia (as Nick pointed out), "e-zine" article banks and most US government sites.
There is also something known as "fair use" - this pertains to using a short quote of someone else's work for things like reviews, commentaries, news reports, etc. There is no strict legal definition of "fair use". You can read what the US copyright office has to say about it here
If they licensed their work in such a way that you could use it (probably with conditions), and even if you abode by those, Google would not like your site for duplicate content.
I don't know that because its "open source or something" - I know that because I found this footer at the bottom of the page:
And the Copyrights link leads to a page where it gives more detailed information including the following :
Want to know if/how you can use some other site's content? Read!
Just because you "may" use content from someplace doesnt mean its a good idea. As eholt pointed out, Google doesn't like duplicate content - you're likely to find your pages banished to the very end of the search results.
You'll get the most benefits from your content if you write your own original articles - or hire someone to write for you.
About content copyright 
