What is the difference between res and req?

In the realm of web development and particularly in the context of Node.js, I often hear about 'res' and 'req' being used interchangeably. Could you please clarify for me the fundamental difference between these two terms? Are they simply abbreviations for two distinct concepts, or do they represent different aspects of the same process? Furthermore, how do they specifically relate to handling HTTP requests and responses in a server-side application?