Catalyst is an in-situ framework created as part of ParaView a few years ago. With time and use, we saw the different drawbacks of our approach and then it was decided to create a new architecture, Catalyst API.
With this new implementation, it is:
easier to implement in your simulation (less knowledge required)
easier to update from a version to another (less dependencies, has binary compatibility)
possible to activate Steering mode, where ParaView can modify simulation parameters at runtime
This is documentation for the Paraview implementation of the Catalyst API (Catalyst 2) if you are looking for information regarding the previous version of catalyst check this manual.