“Linux” as a term refers to two things: First, it refers specifically to the Linux kernel. Second, in a broader sense, it refers to the various packagings of the Linux kernel with other programs to provide the functionality required of a complete operating system.
Sound strange? It’s not; it’s one of the things that makes Linux so versatile. The kernel itself manages the hardware, memory, and the other parts of a computer system which are typically opaque to installed programs.
Programs installed to provide additional functionality are referred to as “user-land” or “the user-land.” The combination of kernel and user-land constitute what are referred to as “distributions,” many of which we are familiar: Red Hat Enterprise Linux, Ubuntu, Arch, Fedora, and so on.
In a broad sense, the term “Linux” refers to the operating systems created by the pairing of kernel and user-land, but the term is ambiguous. “Distribution,” on the other hand, refers to the pairing of the kernel with a user-land built to some specification. Ubuntu, for instance, varies quite a bit from CentOS 6. Both of these are separate distributions of Linux.
Unlike operating systems, which are built in a monolithic fashion (where the user-land and kernel are tightly-coupled, such as FreeBSD, VMS, Windows, etc.), Linux allows for variations on theme which number into the thousands. The term “distro” better fits these variations, as each one is not entirely unique from the next (because of the shared kernel) but may differ substantially in terms of the user-land.