tags are basically assigning various keywords or related topics for your post. each tag has its own permalink so if you use 5 tags for a post, your post wil show under 5 different additional permalink pages aside from original category page. you however risk onsite duplicate content which could get you penalized on google and one way to get arounf the onsite duplicate content issue involving tags is canonical url (you can google canonical url for more info about what it is and how it works)