Here are the classes, structs, unions and interfaces with brief descriptions:

[detail level 1234]

spica
BVHPrimitiveInfo
BVHNode
BVHAccel	Binary BVH accelerator class
BucketInfo
Children
Node
ComparePoint
CompareToBucket
SIMDBVHNode
Conductor
Dielectric
Diffuse
Plastic
RoughConductor
RoughDielectric
RoughPlastic
OrthographicCamera	Orthographic camera
PerspectiveCamera	Perspective camera
Accelerator	Interface for intersection test accelerators
ArgumentParser
AxisComparable
Bounds2_
Bounds3_
BSDF
BSphere_
BSSRDF	BSSRDF Interface
SeparableBSSRDF	SeparableBSSRDF class
DiffuseBSSRDF	Diffuse BSSRDF class
SeparableBSSRDFAdapter	Adapter for evaluating surface reflection part of BSSRDF
DiffusionReflectance
DipoleDiffusionReflectance
BxDF	The base class of BxDFs
LambertianReflection	Lambertian refrection
LambertianTransmission	Lambertian transmission
SpecularReflection	Specular reflection (Metal-like effect)
SpecularTransmission	Specular transmission
FresnelSpecular	Fresnel specular refraction (Glass-like effect)
MicrofacetReflection	Microfacet reflaction
MicrofacetTransmission	Microfacet transmission
Camera
CObject
PluginManager
ConstantTexture
Stack
Vector2_	Two-dimensional vector
Vector3_	Three-dimensional vector
Point2_
Point3_
Normal3_
RuntimeException
Film
Filter
Fresnel	The inteface for Fresnel reflections
FresnelConductor	Fresnel conductor class
FresnelDielectric	Fresnel dielectric class
FresnelNoOp	Dammy Fresnel class which returns always one
HashGrid
Image	Image class
Integrator	The integrator interface
SamplerIntegrator	The sampler integrator interface
Interaction
SurfaceInteraction
MediumInteraction
CatmullRom
CatmullRom2D
KnnQuery
KdTree
LightSample
Light	The base class for the lights
SurfaceMaterial
SubsurfaceMaterial
Material
Matrix4x4	Matrix of 4 x 4 size
Medium
MediumInterface
ShapeGroup	Shape with materials
MicrofacetDistribution	The base class for microfacet distributions
TrowbridgeReitzDistribution	Trowbridge and Reitz's microfacet distribution (GGX)
BeckmannDistribution	Beckmann microfacet distribution
MipMap
AtomicDouble
PhaseFunction
HenyeyGreenstein
Primitive
GeometricPrimitive
Aggregate
Quaternion	Quaternion
Random	Random number generator with Mersenne twister
RandomQueue	Queue of random-order popping
Ray	Ray class
Texture	Texture interface
RenderParams
Sampler	Random sampler class
Distribution1D
Distribution2D
Scene	Scene provides the interface for scene graph
Shape	Abstract shape class
RGBSpectrum	RGB spectrum
TextureMapping2D	Texture mapping interface
UVMapping2D
PlanarMapping2D
Timer
Tmo	Base class for tone mapping operators
GammaTmo	Gamma correction
ReinhardTmo	Reinhard TMO
DragoTmo	Drago TMO
DurandTMO	Durand TMO
Transform	The transformation operator class
Triangle
Triplet_
Triplet_< T, typename std::enable_if< std::is_arithmetic< T >::value >::type >
Uncopyable	Interface class which forbids copy and assignment
UVTexture
VisibilityTester
HDRFilm
LDRFilm
BoxFilter
GaussianFilter
TentFilter
EndpointInteraction
ScopedAssignment
Vertex
BDPTIntegrator	Bidirectional path tracing
DirectLightingIntegrator
GDPTIntegrator
GDPTFilm
IrradiancePoint
OctreeNode
Hierarchy
Octree
HierarchicalIntegrator	Irradiance integrator for subsurface scattering objects
CacheData
CacheQuery
CacheNode
IrradCacheIntegrator	Irradiance caching
IrradCache
PathIntegrator	Unidirectional path tracing
Photon
PhotonMap
PPMProbIntegrator	Progressive photon mapping: a probabilistic approach
PSSSampler	Management class for PSS sampling
PSSMLTIntegrator	Primary Sample Space Metropolis Light Transport
PathSample	Path sample
SPPMIntegrator	Stochastic progressive photon mapping
SPPMPixel
VisiblePoint
VolPathIntegrator
VolPhotoIntegrator	Volumetric photon mapping
VPhotonMap
AreaLight	Area light class
Envmap	Environment mapping
GridDensityMedium
HomogeneousMedium
Halton	Randomized Halton sampler for quasi Monte Carlo
Independent	Independent random sampling with Mersenne Twister
LowDiscrepancySampler	Low-discrepancy sampling with (0,2)-sequence
Disk	Disk class
Sphere	Sphere class
RenderWorker	Render workder
SceneParser
Dipole
BitmapTexture
Checkerboard
KillTimer
WorkerTask
WorkerTaskManager