How Revit Works

Revit is different from AutoCAD as it's a modelling program. AutoCAD historically has been a line (vector) based program. Revit is the application you use to build your model (Lego) and then display the deliverables (drawing sheets). You are able to take snapshots (views) as required. It's a BIG database. Controlled using 'Families' (Dynamic Blocks).

Revit uses RVT files for storing BIM models. Parametric objects -- whether 3D building objects (such as windows or doors) or 2D drafting objects -- are called families and are saved in RFA files, and imported into the RVT database as needed. Families do not require programming skills and there are many sources of pre-drawn RFA libraries.

The main difference is that AutoCAD is a general computer-aided design and drafting software used to create precise 2D and 3D drawings, and Revit is software for BIM (building information modelling) with tools to create intelligent 3D models of buildings, which can then be used to produce construction documentation. While AutoCAD is a general drawing tool with broad application, Revit is a building-specific design and documentation solution, supporting all phases and disciplines involved in a building project. Both programs are often used within the same firm and even by the same practitioner.

Alignment vs. Osnaps 

Autodesk Revit offers AutoCAD users new sketching tools like Temporary Dimensions and 

Alignment Guides. Alignment guides are similar to the Object Tracking feature in AutoCAD.

Temporary line and arc extensions, as well as other useful snap points relative to your current position in the model, appear to aid you as you design. Whether you are sketching new objects or inserting components, the appropriate dimensions and guidelines assist placement in the Autodesk Revit model. This is a contrast from the osnap feature of 

AutoCAD which snaps to any entity that fits the current osnap settings. With Autodesk 

Revit, the designer can quickly place doors, windows and other components correctly with the option to easily modify the design later. Doors open in the desired direction and windows are placed at the correct height on the wall. You can place these components in either 2D views or 3D views, whichever is easiest. 

No Command-Line or Layers 

The user interface changes that will be most noticeable to the AutoCAD user are the absence of a command line and the layer control feature. In Autodesk Revit, all building tools and components are presented on a single, easy-to-use toolbar. And the user interface in Autodesk Revit is designed to look and work like other familiar Windows applications, 

making Autodesk Revit easy to learn and fun to use on a daily basis. 

Designers stay more focused on the model rather than the keyboard and command line. Layers are not needed to control the visibility of components. Visibility is controlled through the Category Visibility feature in Autodesk Revit, which works on a per-view basis, comparable to the AutoCAD Freeze/Thaw/On/Off by viewport feature. Colour and line type are also controlled by category. Another advantage Autodesk Revit offers is components that know how to display themselves depending on the view in which they are seen. In Revit, components know to only display a footprint in plan view, while all geometry is shown in the 3D isometric view, for example. 

Single file, Multi-User projects 

AutoCAD users are accustomed to storing and retrieving project data from multiple files. In 

Autodesk Revit, all project data is stored in the single Autodesk Revit project file. This eliminates the need to work with external block and Xref management. Autodesk Revit allows multiple users to work on the same project file and merge their changes with every save, or work on their own versions of the file and then merge changes at any point in the design. 

Intelligent Components 

Dimensioning and alignment locking in Autodesk Revit allow the designer to place building components in relation to others - and keep it that way throughout the design - thereby preserving design intent. Components may be locked together in more than one place to preserve wall alignments, room areas, and door and window placements while the model is being altered in other areas. Component relationships may be constrained and overridden at the designer’s discretion. Dimensions may be locked or editable while the parent objects behave accordingly. Because objects can be aligned and locked into place, many repetitious editing commands are eliminated, allowing for faster work and fewer errors. 

"AutoCAD and Revit are interoperable and can be used together, for example, to incorporate designs created in AutoCAD within a Revit project. Firms may use AutoCAD on some projects or components of a design and use Revit to generate BIM deliverables and to enable collaboration with other design disciplines."

Source: What is the difference between Revit and AutoCAD?

.RVT	The file format of a project model. There are three separate applications for Structure, Architecture, and MEP (Mechanical, Electrical, and Plumbing). Each of these applications saves a standard project file in the.RVT format. Each file can be opened directly in any of the three applications and. RVT files can be linked to and inserted into one another. This is crucial for coordination between disciplines. If groups of elements are created and saved separately, they are also in the.RVT format.
.RFA	The file format of a component Family. These files can be opened directly, inserted, or loaded into an.RVT file
.RTE	The project template file. When a new model is started, an.RTE  file is used to create the model. When you save the file, it is saved as a new.RVT model. The file contains all of your company’s standards and settings. Views, sheets, and families should be set up and ready to use in the project template.
.RFT	The family template file. When a new family is started, an .RFT   file is used to create the family. You can use a default or custom template. When saved, the file becomes new.RFA family. You can customize the family template to include commonly used reference planes, parameters, etc.