Site map is a structured representation of the content. In other words, site maps are used to visualize Information Architecture of a website or mobile app. As a tool, site maps are more often used for websites. I strongly recommend using site maps even for small projects, though some people think it is not necessary. I believe that laying out all content in a structured way really helps to optimize the Information Architecture and makes it easier to people to navigate your website or mobile application. I am not saying that you should go extreme and create a site map for all 2-3 pages website, it’s all project-specific, so use your own judgement. Look at the example site map of a medium-sized website:
As lots of UX designers, I am a huge fan of using sticky notes when working on a site map, especially for big projects. Visual representation of a website’s tree structure with all the content sections right on my wall makes it easier for me to move stickies around in order to get the optimal Information Architecture.