A dichotomy is an idea or classification split in two. When you point out a dichotomy, you draw a clear distinction between two things.
A dichotomy is a contrast between two things. When there are two ideas, especially two opposed ideas — like war and peace, or love and hate — you have a dichotomy. You often hear about a "false dichotomy," which occurs when a situation is unfairly represented as an "either/or" scenario. For example, the statement "All cars are either small and efficient or large and polluting" creates a false dichotomy because there are some cars that don't fit into either category.