Loaders do the pre-processing transformation of virtually any file format when you use sth like require("my-loader!./my-awesome-module") in your code. Compared to plugins, they are quite simple as they (a) expose only one single function to webpack and (b) are not able to influence the actual build process.
Plugins on the other hand can deeply integrate into webpack because they can register hooks within webpacks build system and access (and modify) the compiler, and how it works, as well as the compilation. Therefore, they are more powerful, but also harder to maintain.
