OWA, (Outlook Web App), is the current name for the webmail service of Microsoft Exchange Server 5.0 and later that comes included as a part of Microsoft Exchange Server or Microsoft Office 365. It was previously called "Outlook Web Access."
OWA is used to access e-mail, calendars, contacts, tasks, and other mailbox content, like the Outlook application. Depending on the version of Exchange and SharePoint, access to SharePoint documents may be available through OWA.
As you might expect, functionality and features vary widely based on the version, with more modern versions having more functionality and more advanced features.