I'm new to building content and I keep reading about "the duplicate content penalty."
Assume that I have 10 major categories with about 10 articles (length = 400+- words) in each category. I had planned to have a main page for each category which would have the first 50 words of each article and a link to the article.
Is this a bad concept? Is the excerpt in the link considered "duplicate content?"
I don't think you'll have any problems with this. Duplicate content won't cause your web site to get banned unless all or near all of your web site is duplicate content. As long as that's not the case the worst thing that could happen is some of your web pages being downgraded in the ranking, but like I said I don't think you'll have any problem with it.