Web applications are applications that are accessed over the "web", which can mean the Internet, or an internal network (an intranet).
In a web application, the user's browser functions as a client.
The heavy-lifting is done by code on the server side. Web applications follow the client-server model in general. Some examples of web applications include Google Calendar, Zimbra, and Office Outlook Web Access.
Rich Internet Application (RIA)
These applications offer a rich amount of interaction possibilities using their GUI. Rich internet applications tend to be more alike desktop applications than static web-pages. A RIA for example allows to interact using drag and drop, 3D effects animations and other media.