What are Facets?
Facets are a feature of IntelliJ IDEA that do not have an *direct *counterpart in Eclipse (although they are similar somewhat to Eclipse Natures. See question below). Facets have been introduced to streamline and simplify the configuration of a project by packaging all the settings required to reference a particular framework or technology. For example, to enable Spring in your project, you only have to add the corresponding Facet.
When processing a Facet, IntelliJ IDEA downloads and configures all required libraries. In addition, IntelliJ IDEA recognizes the code model, making referencing the framework a hassle-free operation.
How do I manage Facets in IntelliJ IDEA?
You can add and remove facets by opening the Project Structure dialog box and clicking on Modules. Next, the +button gives you a chance to locate and add existing facets. Instead, by selecting Facets and acting next on the -button you can remove existing facets.
In some cases, you can add more than one facet of the same type to your project. For example, you can have multiple Web facets for deploying the application to different servers, or several EJB facets targeting different versions of EJB. In particular, a module can have at most one Android facet.
Are Facets similar to Eclipse Natures?
Yes, Eclipse natures are in some way close to facets. For example, the Eclipse's Android nature refers to settings made available to the Android plugin. IntelliJ can read the nature format and transform detected settings into a facet.