A .csproj file, which stands for C# Project, is an XML-based project file format used by Microsoft's .NET development platform, primarily within Visual Studio and the .NET Core/SDK tooling. It contains crucial metadata necessary for the compiler and build system to correctly build a software project. This metadata includes references to source code files (like .cs files), configuration settings for the compiler (such as target framework, output type like 'Library' or 'Exe'), dependencies on external NuGet packages, build targets, and custom build steps. Essentially, the .csproj file defines how a set of source code files should be compiled into an executable, library, or web application. Modern .csproj files, especially those used with .NET Core and later, follow a simplified SDK-style format, making them easier to read and manage compared to older, more verbose formats. They are fundamental to any C# or F# project managed by the MSBuild system.