I just finished an article for Headphonesty on headphone amplifier basic terms. It focuses on building block concepts and aims to make technical marketing copy more comprehensible by users without an electronics background.
Here are the topics covered:
- Single-ended and push-pull
- Classes of amplification (A, B, AB)
- Output coupling methods (direct, capacitor, transformer)
- Amplifying devices (tubes, transistors, ICs)
- Negative feedback
This article is probably a good introduction to articles on WTF Amps like the Output Stages page or the various headphone amplifier projects.