A file is the common storage unit in a computer, and all programs and data are "written" into a file and "read" from a file. A folder holds one or more files, and a folder can be empty until it is ...
A filesystem is a way that an operating system organizes files on a disk. These filesystems come in many different flavors depending on your specific needs. For Windows, you have the NTFS, FAT, FAT16, ...