Modules are the building blocks of UE4. The engine is implemented as a large collection of modules, and games supply their own modules to augment them. Each module encapsulates a set of functionality, and can provide a public interface and compile environment (with macros, include paths, and so on) for use by other modules.
Modules are declared through C# source files with a .build.cs extension, and are stored under your project's Source directory. The C++ source code belonging to a module is stored next to to the .build.cs file, or in subdirectories of it. Each .build.cs file declares a class deriving from the ModuleRules base class, and sets properties controlling how it should be built from its constructor. These .build.cs files are compiled by UnrealBuildTool and constructed to determine the overall compile environment.
The typical structure for a .build.cs file is as follows.
public class MyModule : ModuleRules
public MyModule(ReadOnlyTargetRules Target) : base(Target)
// Settings go here