IDA provides a single way to install plugins. Just stick them under plugins and you're good to go.

While it works, the drawbacks are many.

1. You have to copy your plugins into the plugins directory. For Python plugins, this can get quite cumbersome;
2. Plugins often depend on multiple files. This has the tendency to clutter your plugins directory quite a bit;
3. The same plugins are loaded for all users and projects. This is not always desireable;
4. Adding a new plugin requires root access.

The plugin-loader plugin allows you to circumvent all those drawbacks.

Once installed (the old way) the plugin_loader.py plugin allows you to define plugin-lists. There are 3 types of lists - system-wide, user-specific, and project-specific. Listed plugins load automatically at the appropriate time.

All plugin lists are named plugins-7.1.list (7.1 being the IDA version), and look something like this:

C:\Plugins\my_plugin.py

# This is a comment. Comments are always entire lines.
C:\OtherPlugins\another_plugin.py

When IDA starts, both my_plugin.py and another_plugin.py will be loaded.

The system-wide list resides under IDA’s cfg subdirectory. The path can be found using idaapi.idadir(idaapi.CFG_SUBDIR). This list requires root access to modify as it is in IDA’s installation directory.

Located in IDA’s user-directory. $HOME/.idapro on Linux, %APPDATA%/HexRays/IDA Pro on Windows. The path can be found using idaapi.get_user_idadir(). Users can set their own plugins to load, thus eliminating the need for root access.

Located in the same directory as the .idb you're loading.

To install your plugins, just add them to one of the lists. This allows you to easily update plugins as you go without ever needing to copy them.

When IDA starts, the plugin lists the locations of plugin lists in the output window.

[PluginLoader] Loading plugins from:
[PluginLoader]   System-wide List:      C:\Program Files\IDA 7.1\cfg\plugins-7.1.list
[PluginLoader]   User-specific List:    C:\Users\user\AppData\Roaming\Hex-Rays\IDA Pro\plugins-7.1.list
[PluginLoader] Failed creating system plugin list at C:\Program Files\IDA 7.1\cfg\plugins-7.1.list
[PluginLoader] Created user plugin list at C:\Users\user\AppData\Roaming\Hex-Rays\IDA Pro\plugins-7.1.list

Since IDA is not running as admin - it cannot create the system plugin list.