Here is some brief note about the section in which you can see lots of improvement in Odoo v12.

1. Usability

2. Documents

3. Barcode

4. IoT

5. Inventory

6. Purchases

7. Manufacturing

8. Discuss

9. Activities

10. eSign

11. Fleet

12. Leaves

13. Email Marketing

14. CRM

15. Events

16. Point of Sale 

17. Sales

18. Website

19. eCommerce

20. Timesheets and Forecast

21. Project

22. Expenses

23. Helpdesk

24. Studio

25. Invoicing

26. Accounting

27. Localizations

28. Subscriptions

29. Mobiles

30. Odoo.sh

https://www.odoo.com/odoo-12-release-notes is the full release note 

Please check out the overview of Odoo v12 given by the Fabien Pinckaers.