The Deterministic Random Bit Generator (DRBG) is a collection of cryptographically secure random bit generators and modifiers written to the NIST 800-90 specification. Namely, the HMAC and Hash generators are implemented and pass the known answer tests (KATS) while the CTR based generator exists with the intent of matching spec but does not pass KATS for unknown reasons (mis-interpretation of KATS or otherwise).
The combinators allow users to combine two or more generators for new desired effects, such as XORing two generators together, using one generator to reseed another (for obtaining a longer lifetime, presumably), and precomputing randoms in batch (buffering).