The CASA NRAO package consists of the casacore package and NRAO/ALMA .CASA package. This split was made to get a better separation of core libraries and applications. CASA is built on top of casacore.

The system consists of a set of layered libraries (packages) which can be seen best on the Modules page.
The main features of CASAcore are:

scimath package (libcasa_scimath) : N-dim functions with auto-differentiation and linear or non-linear fitting.
tables package (libcasa_tables) : Table data system supporting N-dim arrays with advanced querying.
measures package (libcasa_measures) : Values in astronomical reference frames using physical units (Quanta).
ms package (libcasa_ms) : MeasurementSets for storing data in the UV-domain.
images package (libcasa_images) : N-dim images in world coordinates with various analysis operations.

Casacore notes describe some parts of the system in more detail.

CASA consists of the following subpackages:

casa	Core modules: N-dim arrays, quanta, OS, IO, logging, and various other classes.
scimath	Mathematical modules N-dim functionals, linear/non-linear fitting, and miscellaneous,
tables	Database-like tables with advanced query language (TaQL).
measures	Quantities with references frames and their persistency.
lattices	Memory- or disk-based N-dim arrays (lattices) with masking, regions, expressions, and math.
fits	A C++ interface on top of cfitsio.
ms	The data format for visibility data as described in the MeasurementSet definition.
msfits	Mapping of MeasurementSets to/from FITS.
coordinates	Coordinates for astronomical images.
components	Sky components (point sources, gaussians, etc.)
images	N-dim images with masks, coordinates, and history.

NRAO & ALMA modules:

alma

atmosphere

atnf

calibration

casadbus

casaqt

display

msvis

nrao

plotms

simulators

synthesis

tableplot

xmlcasa

Detailed build instructions are given on the casacore pages.
When using a specific casacore package, it is useful to know the package dependencies when linking a program. They are as follows:

casa
scimath	casa
tables	casa
measures	tables scimath casa
lattices	tables scimath casa
fits	measures tables scimath casa cfitsio
ms	measures tables scimath casa
msfits	ms fits measures tables scimath casa cfitsio
coordinates	fits measures tables scimath casa wcslib cfitsio
components	coordinates fits measures tables scimath casa wcslib cfitsio
images	components coordinates fits measures tables scimath casa mirlib wcslib cfitsio

mirlib is distributed and built as part of casacore. wcslib and cfitsio are external packages that have to be installed before building casacore.