The portal name is the identity of the user's help desk account in Zoho Desk. Typically, it is the company's name, and it is created when the user signs up for the help desk. Departments are divisions categorized within an organization. It can be based on products, teams, locations, etc. Every portal can have multiple departments. 

Agents can switch between portals and departments from the Ticket Views page by tapping on Change. They can also search for the departments using the search icon in the side bar.

In the above video, the highlighted part is the portal and the others are the departments configured in that portal.