A '.py' file is a source code file written in the Python programming language. Python is a high-level, general-purpose programming language known for its readability and versatility. Python scripts contain instructions that are executed by a Python interpreter. These scripts can range from simple one-line programs to complex applications with thousands of lines of code. They are used in a wide variety of domains, including web development, data science, machine learning, scripting, automation, and scientific computing. The Python language emphasizes code readability, and its syntax allows programmers to express concepts in fewer lines of code than would be possible in languages such as C++ or Java. Python supports multiple programming paradigms, including object-oriented, imperative, and functional programming styles. Python scripts are typically plain text files that can be edited with any text editor or integrated development environment (IDE).