Outlook Web App originally called Outlook Web Access and before that Exchange Web Connect is a webmail service of Microsoft Exchange Server 5.0 and later. Outlook Web App comes as a part of Microsoft Exchange Server or Microsoft Office 365.
Outlook Web App is used to access e-mail calendars, contacts, tasks, documents and other mailbox content when access to the Microsoft Outlook desktop application is unavailable. In the Exchange 2007 release, OWA also offers read-only access to documents stored in Microsoft SharePoint sites and network shares. Microsoft provides Outlook Web App as part of Exchange Server to allow users to connect remotely via a web browser. Some of the functionality in Outlook is also available in this web "look-alike". The most important difference is that Microsoft Outlook allows users to work with e-mail, calendars, etc., even when an internet connection is unavailable, whereas OWA requires an internet connection to function.