What is TAB in Salesforce?

Custom tabs let you display custom object data or other web content in Salesforce. When you add a custom tab to an app in Salesforce Classic, it displays as a tab. When you add a custom tab to an app in Lightning Experience, it displays as an item in the app’s navigation bar.
Custom tabs display custom object data or other web content embedded in the app. You can create any of these types of custom tabs.
Custom Object Tabs

• Custom object tabs (available only at an app level and not on subtab apps) display the data of your custom object. Custom object tabs look and function just like standard tabs.

Web Tabs

• Custom web tabs display any external web-based application or web page. You can design web tabs to include the sidebar or span across the entire page without the sidebar.

Visualforce Tabs

• Visualforce tabs display data from a Visualforce page. Visualforce tabs look and function just like standard tabs.

Lightning Component Tabs

• Lightning component tabs make Lightning components available in the Salesforce mobile apps and in Lightning Experience. Lightning components aren’t supported in Salesforce Classic.

Lightning Page Tabs

• Lightning page tabs let you add Lightning app pages to the Salesforce mobile app and Lightning Experience navigation bars.
• In Salesforce Classic, Lightning page tabs don’t display on the All Tabs page when you click tab plus icon. Lightning page tabs also don’t appear in the Available Tabs list when you customize the tabs for your apps.