The definition of a "classic" can be a hotly debated topic. Depending on what you read, or the experience of the person you question on the topic, you may receive a wide range of answers. So, what is a "classic"--in the context of books and literature?
A classic usually expresses some artistic quality--an expression of life, truth, and beauty.
A classic stands the test of time. The work is usually considered to be a representation of the period in which it was written; and the work merits lasting recognition. In other words, if the book was published in the recent past, the work is not a classic.
A classic has a certain universal appeal. Great works of literature touch us to our very core beings--partly because they integrate themes that are understood by readers from a wide range of backgrounds and levels of experience. Themes of love, hate, death, life, and faith touch upon some of our most basic emotional responses.
A classic makes connections. You can study a classic and discover influences from other writers and other great works of literature. Of course, this is partly related to the universal appeal of a classic. But, the classic also is informed by the history of ideas and literature--whether unconsciously or specifically worked into the plot of the text.