Both the terms theme and topic play a unique and independent role in writing. Theme generally refers to the central idea or a perception conveyed in writing. A topic is the subject that is treated or presented within writing. Topics play the role of explaining what a story is all about, while, themes explain the reason why it was written in the first place. The two terms can be confusing, especially to students when asked by their teacher to write essays based on a certain theme with a particular topic. Many feel the two terms should be used interchangeably, but the fact is they are two totally different concepts used in writing.
