From the very beginning, QGIS was developed with the extensible and modular architecture. Originally, it allowed us to extend its functionality only with C++ plugins. But, starting from version 0.9, when Python support was added, users and developers were able to create plugins using the Python programming language as well.
Every QGIS Python plugin is just a set of the Python modules and additional files bundled into a single Python package. These packages should be placed in separate subdirectories under a special directory in the QGIS home path. Usually, this is ~/.qgis2/python/plugins, where ~ is a user home (profile) directory. In Windows, it is C:\Users\your_profile, and in UNIX-like systems, it is /home/your_profile.
The minimal working plugin should contain two files in this directory: