All operating systems provide a filesystem, a way of mapping a logical abstraction of folders (or directories) and files to the bits and bytes stored on a hard drive or other storage device. As humans, we typically interact with the filesystem using a drag-and-drop interface of folders and files of different types, or with command-line programs such as cp, mv, and mkdir.
As programmers, we have to interact with the filesystem with a series of system calls. You can think of these as library functions supplied by the operating system so that programs can call them. They have a clunky interface with integer file handles and buffered reads and writes, and that interface is different depending on which operating system you are ...
