All the big players, including Microsoft, IBM, Sun, Netscape and Borland, have embraced 3-tier. On the other hand, Netscape is really the champion of crossware. Still, as you will see, crossware may fit neatly in the picture and I'll introduce it as well. By enterprise software, I am referring to applications that supports the core business of the organization. Mission-critical would have been another good name.
Enterprise software is vital for the survival of the organization.
Obviously what qualifies as mission-critical is very specific to each organization. For example, a 3D studio would argue that graphic packages are mission-critical. Still, in most cases, the backoffice is the mission-critical software. The ability to order goods, invoice for products or services, manage stock, etc. is what makes or breaks most businesses. Departmental software is essentially all the rest.