From BlenderWiki

Extending Blender

Unlike many programs you may be familiar with, Blender is not monolithic and static. You can extend its functionalities without having to modify the source and recompile. There are two ways of extending Blender: Python scripting and Binary Plugins.

Python scripts

Outside of the Blender executable, there are literally hundreds of scripts written by many people:

• Several scripts are bundled with Blender.
• Not all scripts can be included within the Blender release for logistical or support reasons, but many of them work reliably and are very useful, so they have been catalogued and are available in this wiki in the Scripts Catalog and in the new Extensions tracker.

Plugins

There are many plugins to extend Blender in two major areas:

• Texture Plugins - Change the way a material appears.
• Video Sequences - Do something neat to a video stream.

The default installation of Blender includes some plugins, but you can find others at Kent Mein’s repository and in the new Extensions tracker.

Saving your own Extensions

Files location

In a GNU/Linux distribution, there are two folders called “scripts” and “plugins” under one of these folders:

• /usr/share/blender/, OR
• /usr/lib/blender/,

depending on your distribution.

In a Windows installation, there are also two folders called “scripts” and “plugins”, but the “scripts” folder is in a directory called “.blender”, which is located alongside the plugins folder. The location of those folders depends on the choice you make during installation:

Use application data directory

In this case scripts and plugins will be in a place like:

C:\Users\****\AppData\Roaming\Blender Foundation\Blender\

Use installation directory

In this case “plugins\” and “.blender\scripts\” will be in the installation directory of Blender.

I have defined a %HOME% variable - please install here

In this case the folders will be at the defined “%HOME%” location.

You can setup a path for scripts and one for plugins in User preferences (User Preferences → File Paths).

Scripts

If you find a script useful for you, download it to your scripts directory and it will be there when you need it.

If it is not in the scripts folder, you have to load it using the text editor window and then File → Run Python Script.

Plugins

If a plugin is in the Blender plugins install directory (or where you choose via User Preferences), you will easily find it when you use Texture or Sequence Plugin options.

If not, you will have to find it on your hard disk.

In both case it will be dynamically loaded for you and begin working immediately.