In Nintex Workflow, if you simply use the Created By or Modified By properties of a SharePoint List Item / Document, you will most likely get a domain\login format.
If this value is something you want to use in an email, notification, document etc, then a domain\login format is not ideal. Instead, you're probably more interested in the Display Name.
There is a little known button that will help you get the Display Name for either the Created By or the Modified By fields. Or, any People/Group field (on your item or another item).
How Do I Get the Display Name?
Firstly, create a Text variable. Give it a name like textDisplayName.
Add a Set a Variable action to your workflow. Open the configuration of this action and select your textDisplayName variable. For the Equals dropdown, select List Lookup.
Even though we are trying to get the property of the current item, we are still doing a List Lookup. Except, we are doing a List Lookup on the Current Item (not another list... but having said that, this will work on a lookup on an item in another list).
Once Current Item is selected, you'll be able to click on the next dropdown, to select which field you want. In our case, either the Created By or Modified By fields.
This is where it gets interesting. Notice the little button to the right of the Created By/Modified By that you selected. It has a ... in it.
As soon as you click on the ... button, this lets you select the Return Type. When you select Display Name, you'll notice your field will change to "Created By Display Name".
But that's not all. When you do click on that ..., you get other options too. To keep that in mind.
Notice that you can get the Email Address, Login Name and User ID also. This opens up a bunch of possibilities for your workflows. So keep this little nugget in mind, the next time you need these types of values and it'll save you from having to query User Profiles or Active Directory.