"A very basic way of thinking about literary theory is that these ideas act as different lenses critics use to view and talk about art, literature, and even culture. These different lenses allow critics to consider works of art based on certain assumptions within that school of theory. The different lenses also allow critics to focus on particular aspects of a work they consider important.
For example, if a critic is working with certain Marxist theories, s/he might focus on how the characters in a story interact based on their economic situation. If a critic is working with post-colonial theories, s/he might consider the same story but look at how characters from colonial powers (Britain, France, and even America) treat characters from, say, Africa or the Caribbean. "
- Purdue Online Writing Lab, Literary Theory & Schools of Criticsm