Quick descriptions of the packages:

audio -
    Audio API for panda
builder - 
    A tool to create Geoms and GeomNodes from a pile of unconnected
    triangles or polygons.  It separates geometry according to state
    (texturing, normals, etc.), discovers connections where they exist
    and tries to build nearly-optimal tristrips.
chan -
    Animation channels.  This defines the various kinds of
    AnimChannels that may be defined, as well as the MovingPart class
    which binds to the channels and plays the animation.  This is a
    support library for char, as well as any other libraries that want
    to define objects whose values change over time.
chancfg - 
    This package contains the code for the 'channel configuration' system.
    This is responsible for asking for a window to be opened, possibly binding
    'pipe video' channels to it, and dividing it up into Display Regions.
chat -
    Implementation of a text-entry system along the bottom edge of the
    window, along with any other support functions for an online chat
    system that might be required.
collide -
    This package contains the classes that control and detect collisions
configfiles -
    This package contains the housekeeping and configuration files needed by
    things like attach, and emacs.
cull -
    This package contains the Cull Traverser.  The cull traversal collects all
    state changes specified, and removes unneccesary state change requests.  Also
    does all the depth sorting for proper alphaing.
device -
    Device drivers, such as mouse and keyboard, trackers, etc...  The 
    base class for using various device APIs is here.
dgraph -
    Defines and manages the data graph, which is the hierarchy of
    devices, tforms, and any other things which might have an input or
    an output and need to execute every frame.
display -
    Abstract display classes, including pipes, windows, channels, and
    display regions
doc -
    Documentation that doesn't fit in any of the packages
downloader -
    Tool to allow automatic download of files in the background
dxgsg - 
    Handles all communication with the DirectX backend, and manages state
    to minimize redundant state changes.
effects -
    Various graphics effects that aren't shaders.  I.E Lens Flares
egg -
    A.k.a. the "egg library", this reads, writes, and manipulates egg
    files.  It knows nothing about the scene graph structure in the
    rest of the player; it lives in its own little egg world.
egg2sg -
    A.k.a. the "egg loader", this converts the egg structure read from
    the egg library, above, to a scene graph structure, suitable for
    rendering.
event -
    Tools for throwing, handling and receiving events
framework -
    A simple, stupid framework around which to write a simple, stupid
    demo program.  Handy for quickly writing programs that open a
    window and display the OmniTriangle.
glgsg -
    Handles all communication with the GL backend, and manages state
    to minimize redundant state changes.
glidedisplay - (defunct)
    This package contains the code to manage pipes/windows/channels on glide
    hardware.
glidegsg - (defunct)
    This package contains the GSG for the glide backend.  All glide calls
    should live in here.
glutdisplay -
    Glut specific display classes.  This uses the Glut library to open
    a window and manage keyboard and mouse events.  Glut has the
    advantage of being widely ported, but unfortunately it needs to
    control the main loop and so limits the kind of programs you can
    write.
glxdisplay -
    X windows display classes that replace Glut functionality.
gobj -
    Graphical non-scene-graph objects, such as textures and geometry primitives
graph -
    Defines the basic graph operations that are not specific to scene
    graphs.  This includes nodes, relations, transitions, and
    attributes.
gsg -
    The base definition for the GraphicsStateGuardian.  This is the
    abstract base class for all backend specifications.
gsgbase -
    Base GSG class defined to avoid cyclical dependency build
gsgmisc -
    Some utility functions for gsg that could not live in the same
    directory for circular dependency reasons.
ipc -
    This package contains the abstraction/adaptation layer for: mutexes,
    semiphores, condition variables, threads, and files.
light -
    Lights needed to be their own package to avoid multiple inheritance
    problems.  All concrete light types are both lights and nodes.
linmath -
    Linear algebra library.
loader -
    Tool for loading various kinds of files into Graph structures.
    Can be done asynchronously
mathutil -
    Math utility functions, such as frustum and plane
net -
    Net connection classes
panda -
    Builds the libpanda shared library.  This is a single library that
    encapsulates most of the packages in Panda, especially those that
    are considered essential to Panda's basic functionality.  On
    Windows platforms, the individual packages are not themselves
    built into shared libraries; the single LIBPANDA.DLL is the only
    library file.  On Unix platforms, the individual packages are each
    built into their own shared library files, and a trivial
    libpanda.so is built that unifies all of them.
pandadx -
    As above, for the DirectX libraries.  This includes all of the
    packages, in addition to those in libpanda, that are required for
    rendering on a DirectX platform.  This also includes
    Windows-specific libraries.
pandaegg -
    As above, for the egg reader.  This includes all of the packages,
    in addition to those in libpanda, that are required for reading
    egg files into the scene graph.
pandagl -
    As above, for the OpenGL libraries.  This includes all of the
    packages, in addition to those in libpanda, that are required for
    rendering on an OpenGL platform, with the exception of Glut.
pandaglut -
    As above, for the OpenGL Glut libraries.
pandaphysics - 
    As above, for the physics/particle systems libraries.
pandarib -
    As above, for the Renderman non-realtime renderer.
particlesystem -
    Tool for doing particle systems.  Contains various kinds of particles,
    emiters, factories and renderers.
physics -
    Base classes for physical objects and forces.  Also contains the
    physics manager class
pnm -
    A more-or-less intact version of the NetPBM package, compiled as a
    single library.  This is a support library for pnmimage.
pnmimage -
    Reads and writes image files in various formats, by using the pnm
    and tiff libraries.
pstats -
    Package for gathering performance statistics
ps2display - (defunct)
    Playstation 2 display classes.
p2gsg - (defunct)
    Play station 2 specific rendering backend.
ribdisplay -
    RIB-specific "display" classes.  These don't actually open
    windows, but actually specify filenames.
ribgsg -
    The RIB-specific rendering backend.  Writing a frame to this
    backend causes a RIB file to be produced, which can be used to
    render the scene offline.
sgattrib -
    Scene graph attributes and transitions (state information)
sgidisplay - 
    This package contains the code to manage pipes/windows/channels on SGI
    hardware.
sgiglutdisplay - 
    This package contains the code to manage pipes/windows/channels on SGI
    hardware for GLUT
sgiglxdisplay -
    SGI specific X windows display classes that replace Glut functionality.
sgmanip -
    High-level tools for manipulation of scene graphs.  This primarily
    defines NodePath, the principle interface for high-level scripting
    languages into the scene graph.
sgraph -
    Graph operations and nodes that are specific to render-type scene
    graphs.  This includes GeomNode, Camera, etc.
sgraphutil -
    Handy utility functions for working with scene graphs.
shader -
    Shaders that generate special effects by modifying the render
    traversal and computing multiple passes.
stats - (defunct)
    Package for gathering performance stats
statsdisplay - (defunct)
    Package for remotely displaying the perfomance stats on a running program
switchnode -
    Package of nodes that switch out geometry underneath them based on
    various conditions.
testbed -
    C test programs, that primarily link with framework.
text - 
    Package for generating renderable text using textured polygons.
tform -
    Data transforming objects that live in the data graph and convert
    raw data (as read from an input device, for instance) to something
    more useful.
tiff -
    Another support library for pnmimage.
util -
    This package contains all the little fiddley things that are atomic to
    just about all the code.  Things like: config, ref count, notify, etc.
vrpn - 
    Defines the specific client code for interfacing to the VRPN API.
wdxdisplay -
    Windows DirectX specific display classes
wgldisplay -
    Windows OpenGL specific display classes