Here is a list of all files with brief descriptions:

AABB.cpp
AABB.hpp	Declaration of the AABB class and free functions that return instances of it
AllocatedArray.hpp
ArrayList.hpp
BlockAllocator.cpp
BlockAllocator.hpp
Body.cpp
Body.hpp	Declarations of the Body and BodyDef classes, and free functions associated with them
BodyConstraint.cpp
BodyConstraint.hpp
BodyType.hpp
Box2D.hpp
BroadPhase.cpp
BroadPhase.hpp
ChainShape.cpp
ChainShape.hpp
CircleShape.cpp
CircleShape.hpp
CodeDumper.cpp
CodeDumper.hpp
CollideShapes.cpp
CollideShapes.hpp
Collision.cpp
Collision.hpp	Structures and functions used for computing contact points, distance queries, and TOI queries
Contact.cpp
Contact.hpp
ContactFeature.hpp
ContactSolver.cpp
ContactSolver.hpp
Defines.hpp
Distance.cpp
Distance.hpp
DistanceJoint.cpp
DistanceJoint.hpp
DistanceProxy.cpp
DistanceProxy.hpp
DynamicTree.cpp
DynamicTree.hpp
EdgeShape.cpp
EdgeShape.hpp
Fixed.hpp
Fixture.cpp
Fixture.hpp	Declarations of the Filter, FixtureDef, and Fixture classes, and free functions associated with them
FixtureProxy.hpp
FrictionJoint.cpp
FrictionJoint.hpp
GearJoint.cpp
GearJoint.hpp
GrowableStack.hpp
IndexPair.hpp
Island.cpp
Island.hpp
Joint.cpp
Joint.hpp
List.hpp
Manifold.cpp
Manifold.hpp
MassData.cpp
MassData.hpp	Declaration of the MassData structure and associated free functions
Math.cpp
Math.hpp
MotorJoint.cpp
MotorJoint.hpp
MouseJoint.cpp
MouseJoint.hpp
PolygonShape.cpp
PolygonShape.hpp
PositionConstraint.hpp
PositionSolverManifold.cpp
PositionSolverManifold.hpp
PrismaticJoint.cpp
PrismaticJoint.hpp
Profile.hpp
PulleyJoint.cpp
PulleyJoint.hpp
RayCastInput.hpp
RayCastOutput.cpp
RayCastOutput.hpp
RevoluteJoint.cpp
RevoluteJoint.hpp
Rope.cpp
Rope.hpp
RopeJoint.cpp
RopeJoint.hpp
SeparationFinder.cpp
SeparationFinder.hpp
Settings.cpp
Settings.hpp	Types and default settings file
Shape.cpp
Shape.hpp
ShapeSeparation.cpp
ShapeSeparation.hpp
Simplex.cpp
Simplex.hpp
SimplexEdge.hpp
Span.hpp
StackAllocator.cpp
StackAllocator.hpp
StepConf.cpp
StepConf.hpp
StepStats.hpp
TimeOfImpact.cpp
TimeOfImpact.hpp
Timer.cpp
Timer.hpp
UnitVec2.cpp
UnitVec2.hpp
Vec2.hpp
VelocityConstraint.cpp
VelocityConstraint.hpp
VertexSet.hpp
WeldJoint.cpp
WeldJoint.hpp
WheelJoint.cpp
WheelJoint.hpp
Wider.hpp
World.cpp
World.hpp	Declarations of the World class and associated free functions
WorldCallbacks.cpp
WorldCallbacks.hpp
WorldManifold.cpp
WorldManifold.hpp