GNU/Linux is a 'Unix-like' operating system because it was based on Unix, and is similar in many ways.
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.
GNU/Linux, and the BSD descendants follow design and operation principles largely similar to UNIX.
Linux comes closest to mind.
The question is rather vague; the applications used in Unix are similar or identical to those used in other operating systems, such as Windows.
Commands you use in a Unix based computer OS to achieve certain things. Similar to MS/DOS commands in Windows. Mostly used in computers running the Linux OS. unix command
You access Unix the same or similar way that you access any other Operating System; you provide a username and a password and log in. From there, you use the shell interpreter to tell it what to do (in a similar way from Windows, for example).
'cat' is short for concatenation; it is a Unix utility program to print the contents of 1 or more files on the standard output. It is similar to the 'type' command in Windows.
The reason for the exec functions being same/similar is because the Mac OS has its roots in variants of the Unix kernel.
vxworks is used to control network and communication devices. vxworks is similar to unix but it uses less processing power than unix.
Systems Administrator :)
Unix in really an umbrella term for a whole category of operating systems that may share some similar code-base. Originally developed in the late 70's by Bell labs, Unix has been forked and re-factored many times by many companies. BSD is a fork of the original Unix system that is currently maintained by Berkley State University, CA. Other Unix forks come from SCO, Oracle, and a few others. On another note, Linux is a free software alternative to Unix, but shares none of the original code.