Here are the data structures with brief descriptions:

Cb2AABB	Axis-aligned bounding box
Cb2BodyDef	A body definition holds all the data needed to construct a rigid body
Cb2BodyEvents	Body events are buffered in the Box2D world and are available as event arrays after the time step is complete
Cb2BodyId	Body id references a body instance. This should be treated as an opaque handle
Cb2BodyMoveEvent	Body move events triggered when a body moves
Cb2Capsule	A solid capsule can be viewed as two semicircles connected by a rectangle
Cb2CastOutput	Low level ray cast or shape-cast output data
Cb2ChainDef	Used to create a chain of line segments
Cb2ChainId	Chain id references a chain instances. This should be treated as an opaque handle
Cb2ChainSegment	A line segment with one-sided collision
Cb2Circle	A solid circle
Cb2CollisionPlane	These are collision planes that can be fed to b2SolvePlanes
Cb2ContactBeginTouchEvent	A begin touch event is generated when two shapes begin touching
Cb2ContactData	The contact data for two shapes
Cb2ContactEndTouchEvent	An end touch event is generated when two shapes stop touching
Cb2ContactEvents	Contact events are buffered in the Box2D world and are available as event arrays after the time step is complete
Cb2ContactHitEvent	A hit touch event is generated when two shapes collide with a speed faster than the hit speed threshold
Cb2CosSin	Cosine and sine pair This uses a custom implementation designed for cross-platform determinism
Cb2DebugDraw	This struct holds callbacks you can implement to draw a Box2D world
Cb2DistanceInput	Input for b2ShapeDistance
Cb2DistanceJointDef	Distance joint definition
Cb2DistanceOutput	Output for b2ShapeDistance
Cb2DynamicTree	The dynamic tree structure
Cb2ExplosionDef	The explosion definition is used to configure options for explosions
Cb2Filter	This is used to filter collision on shapes
Cb2FilterJointDef	A filter joint is used to disable collision between two specific bodies
Cb2Hull	A convex hull
Cb2JointId	Joint id references a joint instance. This should be treated as an opaque handle
Cb2Manifold	A contact manifold describes the contact points between colliding shapes
Cb2ManifoldPoint	A manifold point is a contact point belonging to a contact manifold
Cb2MassData	This holds the mass data computed for a shape
Cb2Mat22	A 2-by-2 Matrix
Cb2MotorJointDef	A motor joint is used to control the relative motion between two bodies
Cb2MouseJointDef	A mouse joint is used to make a point on a body track a specified world point
Cb2Plane	Separation = dot(normal, point) - offset
Cb2PlaneResult	These are the collision planes returned from b2World_CollideMover
Cb2PlaneSolverResult	Result returned by b2SolvePlanes
Cb2Polygon	A solid convex polygon
Cb2PrismaticJointDef	Prismatic joint definition
Cb2QueryFilter	The query filter is used to filter collisions between queries and shapes
Cb2RayCastInput	Low level ray cast input data
Cb2RayResult	Result from b2World_RayCastClosest
Cb2RevoluteJointDef	Revolute joint definition
Cb2Rot	2D rotation This is similar to using a complex number for rotation
Cb2Segment	A line segment with two-sided collision
Cb2SegmentDistanceResult	Result of computing the distance between two line segments
Cb2SensorBeginTouchEvent	A begin touch event is generated when a shape starts to overlap a sensor shape
Cb2SensorEndTouchEvent	An end touch event is generated when a shape stops overlapping a sensor shape
Cb2SensorEvents	Sensor events are buffered in the Box2D world and are available as begin/end overlap event arrays after the time step is complete
Cb2ShapeCastInput	Low level shape cast input in generic form
Cb2ShapeCastPairInput	Input parameters for b2ShapeCast
Cb2ShapeDef	Used to create a shape
Cb2ShapeId	Shape id references a shape instance. This should be treated as an opaque handle
Cb2ShapeProxy	A distance proxy is used by the GJK algorithm
Cb2Simplex	Simplex from the GJK algorithm
Cb2SimplexCache	Used to warm start the GJK simplex
Cb2SimplexVertex	Simplex vertex for debugging the GJK algorithm
Cb2SurfaceMaterial	Surface materials allow chain shapes to have per segment surface properties
Cb2Sweep	This describes the motion of a body/shape for TOI computation
Cb2TOIInput	Input parameters for b2TimeOfImpact
Cb2TOIOutput	Output parameters for b2TimeOfImpact
Cb2Transform	A 2D rigid transform
Cb2TreeStats	These are performance results returned by dynamic tree queries
Cb2Vec2	2D vector This can be used to represent a point or free vector
Cb2Version	Version numbering scheme
Cb2WeldJointDef	Weld joint definition
Cb2WheelJointDef	Wheel joint definition
Cb2WorldDef	World definition used to create a simulation world
Cb2WorldId	World id references a world instance. This should be treated as an opaque handle