Linux is an operating system.
Ubuntu Linux is a sub-distribution/flavour/variety of the Linux operating system and to complicate matters is based on the Debian distribution of Linux ....
Structure is like this
Why so many?
Linux is for hyper nerds like Linus Torvalds (initial developer of Linux) and Mark Shuttleworth (founder of Ubuntu and 1st South African into Space)
Debian is for super nerds.....
Ubuntu is for you and me, and is a user-friendly system evolved from above with GUI's (graphic user interfaces) and designed to operate like Widnows and Mac OSX.
Linux is based on the UNIX OS developed in the '70's and '80's.
The architecture of Linux systems does not allow for 99.9% of scripts/programs to run with out the adminsitrators permission, unlike Windows systems, where .exe., .com and some .dll files can run, without permission, as soon as they enter the system. It is the latter function, that is the origin of most viruses/malware/spyware infections.