An excellent tool if you are looking for a BIM overview.
Definition of BIM :
A Building Information Model is a digital representation of the physical and the functional characteristics of a facility. As such it serves as a shared knowledge resource for information about a facility, forming a reliable basis for decisions during its life cycle from inception onward.
Creating a BIM is different from making a drawing in 2-D or 3-D CAD (see subsequent definitions). To create a BIM, a modeler uses intelligent objects to build the model.
A model that includes three dimensional (3-D) shape information and does not include the 4-D and 5-D characteristics described below.A 4-D BIM that has objects and assemblies that have schedule and time constraint data added to them. The information can be contained in the BIM or can be linked or otherwise associated (integrated and/or interoperable) with project design and construction activity scheduling and time sensitivity estimating and analysis systems.
Building Information Modeling is the human activity of using BIM software and other related software, hardware and technologies to create and use in a building information model.
A 3-D BIM that has objects and assemblies that have a cost dimension added to them. The cost information can be contained in the BIM or can be linked or otherwise associated to the building objects.