Duplicate content ans spam are two different things, but if you take a half article and post in forums - it would be spam with duplicated content.
If you take a half article and place it on your blog with link to it, it won't be spam, but it won't give you any good ranking or similar things.
If you want get something related about ranking, you need to write your own content, unique. Or rewrite someones article in your own words
