After the workflow reaches a stage, it will automatically email any users with that role who have "Receive Role-Based Emails" set to Yes. Sometimes all users will have that set, other times you may only choose one person to receive the emails and for *that user* to assign the work. The email will look something like this - but you can customize it:

In addition, the originator of the workflow (person who entered the workflow) will automatically be notified of any changes to their workflow item.

When the user enters into the application, the first thing they'll notice is the red badge on the tab to tell them they have workflow outstanding:

The entry itself will be highlighted also to lead them to the actions they need to take.

In addition, if they click the home page they'll see a dashboard with details of the outstanding workflow that needs to be processed, along with some statistics on recently submitted items, a calendar of any upcoming reminders, and more.

Scroll down a bit and you'll see the calendar that shows all of your upcoming reminders: 

There are several alternative mechanisms for notifications that can be configured, including customizable HTML email templates, and these will be discussed in a different section.