1. Is Developer Cloud Service and Application Builder Cloud Service are one and the same?
A - no these are two separate services.
Developer Cloud Service (DevCS) - is a platform for your team to collaborate on the development process of any code enabling you to do agile development process, track issues, execute code reviews - as well as automat the DevOps cycle with build automation and continuous delivery.
ABCS - was renamed to VBCS - Visual Builder Cloud Service - it is a visual development and hosting platform for CRUD type of applications.
I would suggest viewing these two demos to understand what the tools do:
2. How do we get ABCS or DCS access? Do we need to buy the subscription separately?
You get DevCS when you license most other Pass services , including VBCS. You can also get trials of both by clicking the try it button on their pages:
3. What are the basic purpose of using ABCS or DCS?
See the answer to A.