Literary criticism seeks to explain, interpret, clarify, and analyze works of literature. Often it increases a reader's understanding of characterization, themes, style, and other literary elements and techniques of a work, an understanding that deepens a reader's knowledge and pleasure, as well. Some criticism is a method of placing a work in not only its own context, but also the context of the culture of its time, thereby broadening a reader's understanding of human nature and history. Such literary criticism that is marked as one of the movements of Postcolonialism, Romanticism, Feminism, Marxism, and the more abstract Deconstruction, Post-Structuralism and Postmodernism can provide new avenues of thinking about certain literary works. With the added "vision" that literary criticism provides, readers glean a better understanding of not only literature, but various facets of experience. Thus, criticism works to deepen readers' understanding and enjoyment of various literary works and their authors as well as enabling good literature to remain alive throughout time to teach readers about life.
Literary criticism has to do with understanding a piece of work more thoroughly. Human understanding occurs in a multitude of different ways, which is why the idea and concept of "literary criticism" has such a wide breadth of applicability. One way to better understand a work might be to hone in on the thoughts and actions of the protagonist. You would then be looking at characterization of the main character, how these qualities are reflected in his or her actions, and how they color your perception of the character. Expanding beyond a purely literary perspective, you might also look into the field of psychology to understand that character on a deeper level. Another way you might approach literary criticism is to evaluate the work in a certain context, like how it speaks to different cultures, if it has any gender issues related to it, or what the work says about its time period. There are a multitude of different ways to approach literary criticism (the ones I described were just examples) but they all serve the common purpose of understanding more about the work.