it's a bit more than what naco said.
A linear plot starts at point A, things happen and it ends at Point Z. Point A was a point of departure and is not revisited.
A circular plot starts at point A goes through point Z and ends up back at point A2. The ending mirrors the beginning only this time the protagonist wins because of the skills and wisdom he has learned on his journey. Or, in a different way, the end echoes the beginning. There are circular elements in Star Wars, ie Anakin Skywalker and Luke Skywalker both have their hands cut off. This is not, technically, a completely circular plot but it has circular aspects in that events in a scene will echo earlier scenes.
A circular plot example would be: Son inherits company from father. At board meeting he is woefully inexperienced and can not keep the board from taking over and replacing him as CEO. He sets out on a quest to become a better man, he learns things, he attains wisdom, and at the end he attends a board meeting where he bests the board and gets his company back. The end mirrors the beginning.
In my opinion, (and thats just mine) I think circular plots are the most complete as they show that the protagonist has changed and that the thing that defeated him in the beginning is now defeated. It shows he has grown and undergone change in a concrete fashion. I find circular plots are more satisfying to me as a reader.