A .lib file is a static library, a collection of pre-compiled routines, subroutines, functions, or objects that can be used in a program. Unlike dynamic libraries (DLLs), static libraries are linked directly into the executable file during the compilation process. This means that the code from the library becomes part of the final executable, making it self-contained and independent of external library dependencies at runtime. Static libraries are commonly used to encapsulate reusable code modules, providing a way to organize and share code across multiple projects. They improve code reusability and reduce code duplication. When a program uses a static library, the linker copies the necessary code from the library into the program's executable file. This results in a larger executable size compared to using dynamic libraries, but it eliminates the need to distribute and manage separate library files. Static libraries are particularly useful when you want to ensure that your program has all the necessary code embedded within it, without relying on external dependencies that might be missing or incompatible on different systems. They are a fundamental part of software development, especially in languages like C and C++.