What's the difference between ACW and PCW?

Hi Gurus,

What's the difference between ACW (Application Controlled Workflow) and PCW (Process Controlled Workflow) ?

I read that SRM 4.0 uses ACW and SRM 7.0 uses PCW.

My current system is SRM 4.0 but I'm upgrading to SRM 7.0.

What is the difference? What are the implications of migrating from ACW to PCW?

Do I have to develop something different?

Thanks in advance