A serial is an ongoing story, written through multiple books. Think about how a soap opera continues from day to day. That is serialized. The stories continue through each piece of the story, but new situations arise for them to tackle. A series is a group of book written around a common theme, whether it's tied together by characters, a location, or a specific theme. There is more closure at the end of each series book. You might read a romance, and another couple is featured. They will then be the main characters in the next book, but couple one's story has been completed.