What is App in Sales force?

An app in Salesforce.com is nothing but a container which contains in it – a name, a logo, and an ordered set of tabs.
All the metadata such as Objects, Visualforce Pages, Classes, etc are independent of an app. An app just helps to group things together visually. But internally the metadata has nothing to do with an app i.e. you can have the same tab, VF Page in multiple apps.

App : It is a group of tabs that work as a unit to provide application functionality. App means application and it is a collection of components with you tabs, reports, dashboards, Vf pages and so on.