An app is a group of tabs that work as a unit to provide FUNCTIONALITY. Users can switch between apps using the Force.com app drop-down menu at the top-right corner of every page.
You can customize existing apps to match the way you work or build new apps by grouping standard and CUSTOM tabs.
NAVIGATION to create an app in Salesforce: Setup ->Build ->Create->App-> Click on new and create your application according to your requirements.