1. Apex
2. Triggers
3. SOQL
4. Order Of Execution
5. Order of Execution in Visualforce page
6. Controller, StandardController and Extensions
7. Coding Standard in Salesforce
8. Debugging Apex code
9. List, Set and Map collection data types
10. Calling Apex method from Custom Button
11. Database.SavePoint and Database.Rollback
12. Unit testing
13. Adding Error message in Visualforce page
14. Visualforce Components
15. Custom Settings
16. Custom Labels
Trailhead with Study guide: https://trailhead.salesforce.com/en/credentials/platformdeveloperii