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. There are 5 types of plugins in Views:

Display
  Display plugins are responsible for controlling where a view lives. Page and block are the most common displays, as well as the ubiquitous 'default' display which is likely what will be embedded.
Style
  Style plugins control how a view is displayed. For the most part they are object wrappers around theme templates.
Row style
  Row styles handle each individual record from a node.
Argument default
  Argument default plugins allow pluggable ways of providing arguments for blocks. Views includes plugins to extract node and user IDs from the URL; additional plugins could be used for a wide variety of tasks.
Argument validator
  Validator plugins can ensure arguments are valid, and even do transformations on the arguments.

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