The difference is in on what you want to get done and what works best for your company.
Do you want to host it on your own servers (Classic) or do you want Adobe to host it for you (Standard)?
There are other differences you should consider too looking through the documentation links available here:
Is there any resource blog/video where I can read about building a triggered workflow? For example when someone subscribes or joins a list, they start to receive automated edms? Thanks!
All content can be found on Adobe Experience League
Hope that helps!