In the context of Cousot and Cousot's abstract interpretation theory, we present a general framework to define, study and handle operators modifying abstract domains. In particular, we introduce the notions of operators of refinement and compression of abstract domains: A refinement enhances the precision of an abstract domain; a compression operator (compressor) can exist relatively to a given refinement, and it simplifies as much as possible a domain of input for that refinement. The adequateness of our framework is shown by the fact that most of the existing operators on abstract domains fall in it. A precise relationship of adjunction between refinements and compressors is also given, justifying why compressors can be understood as inverses of refinements.