This module is not used by GHC itself. Rather, it exports all of the functions and types you are likely to need when writing a plugin for GHC. So authors of plugins can probably get away simply with saying import GhcPlugins.
Particularly interesting modules for plugin writers include CoreSyn and CoreMonad.
Documentation
module CoreMonad
module RdrName
module OccName
module Name
module Var
module Id
module IdInfo
module CoreSyn
module Literal
module DataCon
module CoreUtils
module MkCore
module CoreFVs
module CoreSubst
module Rules
module Annotations
module DynFlags
module Packages
module Module
module Type
module TyCon
module Coercion
module TysWiredIn
module HscTypes
module BasicTypes
module VarSet
module VarEnv
module NameSet
module NameEnv
module UniqSet
module UniqFM
module FiniteMap
module Util
module Serialized
module SrcLoc
module Outputable
module UniqSupply
module Unique
module FastString
module FastTypes