In literature, a linear plot begins at a certain point, moves through a series of events to a climax and then ends up at another point. Also known as the plot structure of Aristotle, it is possible to represent a linear plot line with the drawing of an arc. The primary advantage of using a linear plot is that the reader knows, or at least has an idea, of where the plot goes next, and the reader is guaranteed to get a beginning and ending.
The linear plot forms a contrast with the non-linear plot, which offers the reader several different ways to get from point A to point B. The popular "Choose Your Own Adventure" series of books uses non-linear narratives, as readers make choices at the end of each page, and the choice the reader makes determines the next page that he reads.
In some cases, though, the question has less to do with the opposition of linearity and non-linearity and more to do with the degree to which a particular plot is linear. Non-linear plot lines give the writer more flexibility, such as plot arcs that never find resolution. While a linear plot line is simpler to understand, non-linear plot lines sometimes provide more entertainment.