Linux in informal terms can be either a computer system that:
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complies to the Linux Standard Base (LSB)
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uses the Linux kernel. TODO is this specified by the LSB? where? http://unix.stackexchange.com/questions/75611/what-does-the-linux-standard-base-specify-about-the-kernel. Information specific to the Linux kernel is not covered in this section.
The LSB specifies the minimum interfaces every Linux system must offer.
The central component of the Linux operating system is the Linux kernel, released in 1991 by Linus Trovalds, however much of its core user space software comes from the GNU project. For this reason Linux systems may also be called GNU/Linux.
To be useful to end users directly, the kernel must be packaged with a choice userland utilities. A choice of such set of utilities is called a distribution.