In order to legally be called a "Unix" system, operating systems must undergo a rigorous and expensive certification procedure. "Unix-like" refers to systems that have an architecture similar to Unix, but have not undergone certification.

Unix and MS-DOS are Operating Systems.

Unix-like operating systems are used to save files on a computer operating system. When using this type of system, to unlink a file means to erase or delete it.

Unix and windows are two separate groups of operating systems. Windows is the operating system of about 90% of personal computers, while unix is the basis of many other operating systems, such as Mac OS X

Linux, Minix, Coherent, FreeBSD, etc. These are all clones of Unix