F# is a .NET programming language. F# was initially started as a research project at Microsoft Research Lab by Don Syme. Later, it became a mainstream .NET programming language and is distributed as a fully supported language in the .NET Framework as part of Visual Studio.
According to Microsoft Research, F# is:
A succinct, expressive, and efficient functional and object-oriented language for .NET that helps you write simple code to solve complex problems.
F# is a strongly typed language; it uses the type inference. Since it uses the type inference, programmers need not declare the data types explicitly. The compiler will deduce the data type during compilation. F# will also allow explicit declaration of data types.