Version 2.0 introduced the plugin architecture that is used for almost all the customizable functionality of Smarty. This includes:
Plugins are always loaded on demand. Only the specific modifiers, functions, resources, etc invoked in the templates or PHP scripts will be loaded. Moreover, each plugin is loaded only once, even if you have several different instances of Smarty running within the same request.
Pre/post-filters are a bit of a special case. If a pre/post-filter plugin is found in the plugins directory, it will always be loaded because these filters are never explicitly mentioned in the template language. This also means that the order in which multiple prefilters or postfilters is executed depends on the way they are named (basically, alphabetical order).
There is only one plugins directory (for performance reasons). To install a plugin, simply place it in the directory and Smarty will use it automatically.