User Experience

UX (User Experience) is about making your site usable and useful for your users. A particularly effective flavour of UX is HCD (Human-Centered Design); this puts the human side of things front and center. UI (User Interface) is the specific pieces of the app: the HTML, CSS, and JS.

These are huge topics of their own, but a good starter set of guidelines is Jakob Nielsen’s 10 general principles for interaction design.

1. Visibility of system status
2. Match between system and the real world
3. User control and freedom
4. Consistency and standards
5. Error prevention
6. Recognition rather than recall
7. Flexibility and efficiency of use
8. Aesthetic and minimalist design
9. Help users recognize, diagnose, and recover from errors
10. Help and documentation

Read more details at 10 Usability Heuristics for User Interface Design. The “See Also” section has a few more links with good stuff in.