What are decision tasks?

Basically a task that prompts the user with a question and then proceeds down one of two paths based on their answer. Yes or No.

For example a task called "Review contract" may prompt the user with "Proceed with contract?"

The user would answer 'Yes', 'No' or reject back to the previous task if the contract was missing data.

What does this mean to existing workflows?

Nothing - they will keep working as they always have. You can create new revisions, clone them etc.. Nothing changes.

What does this mean to new workflows?

If you're not going to use decision tasks - Nothing changes. All the tasks in the workflow need to be completed before the workflow is completed. Just as they always have.
If you are going to use decision tasks - a couple of things to keep in mind.

• Decision tasks require both a 'Yes' and 'No' branch.
• All branches must eventually end with an Ending Task.
• The workflow will complete as soon as an Ending node is reached.
• Workflows will likely complete with unfinished tasks.
• Workflow will likely have unreachable tasks (on different branches)

Here you can see that the “Notify parties of the decision” will not be reached since the contract has been approved to be signed.

A tutorial that demonstrates how build the new workflows can be found here