Apps
There is nothing fancy about an "app" in Salesforce, it is just a collection of tabs with a customised logo.
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Hide apps from the profile that are really not necessary - it an appexchange app installs an "app" but it's only got one useful tab that you use once in setup, then hide that app, it just takes extra time to look for the app you really need.
Tabs
Trailhead has a good overview of Tabs https://developer.salesforce.com/trailhead/force_com_introduction/customizing_user_interface/ui_nav_buttons_links