What's the difference between department name and title?

A department name is unique but the department title is non-unique and can be the same across departments. You can't have 2 or more departments with the same name, but you can have multiple departments with the same title.

For example, you can have a Human Resources department (title) that can be common across the enterprise but there can only be one unique US Human Resources department (name) in the enterprise. A department name is mandatory but the title is hidden and optional, by default. You can enable the Title field and make it mandatory.