Categories are a pre-set taxonomy that are applied by selecting a check box. Tags are free-form and are applied by adding text into a blank tag field. So why use one over the other?
Categories
A lot of thought when into making these and are most often used to pull a custom feed into a college website. They broadly define grouping and on the news blog this is typically by college unit, like a department, or initiative, like diversity. Categories are also hierarchal, a department category may be the parent category for level four center categories.
Keep in mind this is shared space. When in doubt about using a category, ask. You don’t want your post showing up in someone’s feed by accident do you?
Tags
Great for more specific description, tags are not hierarchal. We don’t make much use of tags on the public interface to the news blog, some people use them to enhance a custom feed from the blog.