Difference between views handler and views plugin?
The difference is summarized in the first sentence on About Views plugins:


  In Views, a plugin is a bit like a handler, but plugins are not directly responsible for building the query. Instead, they are objects that are used to display the view or make other modifications.


A view handler is implemented using a class derived from the views_handler class, which has methods to build the query, or to alter the result obtained from a query, such as:


views_handler::get_join()
views_handler::pre_query()
views_handler::sanitize_output()
views_handler::post_execute()
views_handler::placeholder()


On the other hand, the views_plugin class has only a method that is related to the query: views_plugin::query().