What is an OWA server? If you have access to a Microsoft Exchange server you usually use Microsoft Outlook on your desktop PC. This connection is through an internal net. If you are on the go you usually are not able to use Microsoft Outlook. There is an extension to Microsoft Exchange server to give access on the internet through a web interface (webmail). This interface is called Outlook Web App.
OWA Webmail uses this web interface to access your mail on your Android device, so you do not have to concern yourself with IT security issues and protocols. The app does not sync anything with your device, all messages, contacts etc. remain on your server.
NOTE: It is not a general webmail client, so you cannot use it for Gmail, Hotmail, Yahoo mail etc.