The casacore package contains the core libraries of the old AIPS++/CASA package. This split was made to get a better separation of core libraries and applications. CASA is now built on top of casacore.
The documentation for casacore can be found at casacore.github.io/casacore.
Various casacore notes are available at casacore.github.io/casacore-notes.
The documentation for python-casacore is available at casacore.github.io/python-casacore.