Since version control systems work best with text, we are looking for file formats that are represented as such. Not that it's impossible to use a version control system with binary files, to my knowledge, you get all the benefits from using it by using mainly text files. For example, a version control system can tell you exactly the differences between two version of the same text file and point you exactly where those differences are but it can only tell you that a binary file has been changed but not how.
So we are looking for design files that are represented in a textual format. As an obvious basic examble, any html/CSS files work. Obviously also, most vector graphic files are written in plain text. For raster images, textual file formats are quite rare.