Revit is Building Information Modeling (BIM) software. Like AutoCAD, Rhino, or SketchUp, Revit can be used to construct a digital model of a building. Unlike these applications, Revit allows elements within a building model to be linked by adjustable parameters. This means that in a Revit model, the behavior of building elements can be mathematically defined; an important consequence is that elements can be linked to each other in such a way that a change to one results in an automatic change to others.
For example, raising the roof of a building in Revit will automatically increase the height of walls whose heights are constrained to the underside of the roof. Or, moving a wall in plan will automatically adjust the lengths of other walls whose endpoints are linked to the first wall.
Families of elements can be defined in Revit, such that changing the definition of a family will automatically change instances of that family throughout the model. For example, a single family of differently-sized windows can be defined, each sharing a common trim design and mullion profile. A change at the family level to the trim design will automatically update all windows in the project based on this family, regardless of their size.
Clearly, the act of constructing a parametric building model transcends the act of constructing a three-dimensional model like ones you may have built in SketchUp, Rhino, or AutoCAD. Revit depends, as these other applications also do, upon a designer’s ability to visualize and work within an on-screen simulated three-dimensional environment. Additionally, the act of creating a parametric building model in Revit requires that a designer be able to intelligently define relationships between and within building elements. It is also true that the successful user of Revit, in addition to understanding how the software works, must understand construction technology sufficiently well in order to intelligently define such relationships.
www.autodesk.com/products/revit-family/overview for information, features overview, and other resources.
knowledge.autodesk.com/support/revit-products for miscellaneous support.
forums.autodesk.com/t5/revit-architecture/bd-p/133 for a discussion forum.