An assembly file is a text file containing low-level programming code written in assembly language. This language is closely related to a computer's machine code and is used to control hardware directly. Assembly files are often used in performance-critical applications, embedded systems, and older technologies like game cartridges. They allow programmers to write instructions that interact directly with a computer's processor, offering more control and efficiency than higher-level languages. However, they are harder to read and maintain, making them suitable mainly for specialized tasks where speed and hardware access are crucial.