What is it?

Here is the official word about Civil 3D:

Autodesk® Civil 3D™ 2004 is the only civil engineering tool that creates intelligent relationships between objects so design changes are dynamically updated. The enhanced 3D interaction gives the user more design flexibility by making it easy to run and visualize what-if scenarios and to correct errors on-the-fly.

Note that the product is officially a preview release. This means it is not fully intended to be used for production yet. The intention is that user learn the new processes and begin creating styles for future production use.