[md] Project structure intro migrated from confluence

tutorials/project_model.md

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 Project model. Project roots and libraries. Configuring the project from code.
-=================
+=================
+
+This topic considers the concept of IntelliJ IDEA projects and related subjects, such as modules, facets, libraries, SDK.
+The project structure and Java classes you can use to manage projects and modules have been considered.
+
+#Project and It's Components
+This section briefly discusses the IDEA project structure, project components and related terms. For more information about projects and their components, refer to
+[Project] (http://www.jetbrains.com/idea/webhelp/project.html),
+[Module] (http://www.jetbrains.com/idea/webhelp/module.html),
+[Library] (http://www.jetbrains.com/idea/webhelp/library.html),
+and
+[Facet] (http://www.jetbrains.com/idea/webhelp/facet.html)
+in
+[IntelliJ IDEA Web Help] (https://www.jetbrains.com/idea/help/intellij-idea.html).
+
+##Project
+In IntelliJ IDEA, a project encapsulates all your source code, libraries, build instructions into a single organizational unit.
+Everything you do in IntelliJ IDEA, is done within the context of a project.
+A project defines some collections referred to as modules and libraries.
+Depending on the logical and functional requirements to the project, you can create a single-module or a multi-module project.
+
+##Module
+A module is a discrete unit of functionality that can be run, tested, and debugged independently.
+Modules includes such things as source code, build scripts, unit tests, deployment descriptors, etc.
+In the project, each module can use a specific SDK or inherit SDK defined on the project level (see the SDK section later in this document).
+A module can depend on other modules of the project.
+
+##Library
+A library is an archive of compiled code (such as JAR files) that your modules depend on.
+IntelliJ IDEA supports three types of libraries:
+
+* **Module Library**: the library classes are visible only in this module and the library information is recorded in the module *.iml file.
+* **Project Library**: the library classes are visible within the project and the library information is recorded in the project *.ipr file or in .idea/libraries.
+* **Global Library**: the library information is recorded in the applicationLibraries.xml file into the <User Home>/.IntelliJIdea/config/options directory. Global libraries are similar to project libraries, but are visible for the different projects.
+
+For more information about libraries, refer to
+[Library] (http://www.jetbrains.com/idea/webhelp/library.html).
+
+##SDK
+Every project uses a Software Development Kit (SDK).
+For Java projects, SDK is referred to as JDK (Java Development Kit).
+SDK determines which API library is used to build the project. If your project is multi-module, the project SDK by default is common for all modules within the project.
+Optionally, you can configure individual SDK for each module.
+For more information about SDKs, see
+[JDKs] (http://www.jetbrains.com/idea/webhelp/jdks.html)
+and
+[Configuring Project JDK] (http://www.jetbrains.com/idea/webhelp/configuring-project-jdk.html)
+in IntelliJ IDEA Web Help.
+
+##Facet
+A facet represents certain configuration, specific for a particular framework/technology, associated with a module. A module can have multiple facets. E.g. Spring specific configuration is stored in a Spring facet.
+For more information about facets see
+[Facet] (http://www.jetbrains.com/idea/webhelp/facet.html)
+and
+[Facet Dependencies] (http://www.jetbrains.com/idea/webhelp/available-facets-and-their-dependencies.html)
+in IntelliJ IDEA Web Help.
+
+#Project Structure
+A project consists of one or several modules.
+Each module includes the plugin source code and so called order entries that refer to SDK and libraries the module uses.
+By default, all modules uses the project SDK.
+In addition, a module can optionally have a set of facets.
+This document explains how you can explore and change the structure of projects using API.
+
+
+