Workflow From Case Comments gives you the ability to create a workflow rule on the Case Comments object. From that rule, you can create a field update that takes effect on either the comment itself or on the parent case.
For example, let's say I have a process whereby I set my case to a status of "Awaiting Customer Response" when I'm waiting for the customer to get back to me. Now my customer logs into the Customer Portal and adds a comment to the case. That constitutes a response, so the system should automatically set my case back to a "Working" status.
All I have to do is write a workflow rule on Case Comment with the requisite criteria (and the criteria can refer to items from the Case as well). Once that's done, I can make a field update that updates the status field on the case.
As people begin creating workflows from Case Comments, they inevitably notice some restrictions about these workflow rules. They ask, what if I want to send an email to the case owner? Why can't the email alert find that field?
As with workflow from Email Messages, Workflow From Case Comments is a "chaining" workflow -- it can trigger other workflow rules. Certain operations on Case Comment are limited because of what the workflow action is "looking at" -- an Email Alert, for example, is "looking at" the Case Comment and so cannot see the Case Owner, for example. However, you can put an email alert on a workflow on the parent case, and that workflow will get triggered and send out the email (and the email will be associated to the parent case, as you might expect).
So, although you can't send an email directly from a Workflow From Case Comments, you can put an email alert on the parent case. So, for example, you might make a Workflow From Case Comments that updates a "Last Comment" field on the parent case with the text of the most recent comment, and then make a workflow on Case which emails out that last comment to the Case Owner, or to certain members of the Case Team.