Here is a list of all topics with brief descriptions:

Core of hydrogen	All major classes of the part of hydrogen
Audio engine	Parts of the source code belonging to the audio engine and coping with the actual audio processing
Audio driver	These classes handle the communication with the sound card of your computer
Data structures	Basic data structures hydrogen uses to store/handle individual instruments, samples, whole drumkits etc
Configuration	Classes and functions allowing for customizing both the behavior and the appearance of hydrogen
Debugging	Classes, functions, and macros making the debugging hydrogen more convenient
Events	Basic classes for communication of the core part of hydrogen with its GUI
MIDI	Various MIDI-related classes and functions
Automation	Various classes for controlling hydrogen from outside the program itself
GUI of hydrogen	All major classes of the GUI of hydrogen
Widgets	Fundamental classes the GUI is constructed with