Outlook Web Access, is the previous 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 is know called Outlook Web App, and typically referred to by the acronym OWA.
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.