The MS-DOS filesystem is very straightforward. It is a 16-bit system based on a File Allocation Table, or FAT16 (FAT for short). The purpose of the file allocation table is to keep track of where to find files on the disk.
In MS-DOS, every DOS based partition has a letter: (A: or B: or C:). Typically, the drive letters A: and B: are reserved for floppy drives. You will most frequently find that the C: drive is the 'bootable partition'. Each drive has a root directory ('\') so the root directory on a given drive looks like this: C:\
Changing drives is as simple as typing the name of the drive letter:
A:> C: <enter>
C:>
MS-DOS then stores files to the system in any arrangement you choose. You can create directories, and store files within those directories. A typical file/path might look like this:
C:\ms-dos\mydir\filename.txt
LIMITATIONS
FAT16 filesystems are compatible with all Microsoft operating systems, but it has severe limitations. First, all files on the system are limited to eight characters and a three letter extension. The MS-DOS filesystem also has a limit of approximately 2.1 Gigabytes owing to the fact that the MS-DOS operating system doesn't recognize 'Int 13' based commands, and therefore cannot issue commands to access the remainder of larger disks.
Keep in mind that MS-DOS is a legacy system kept around for doing command line based work in Windows.