The word "Linux" refers to two things. First, there's the kernel, which handles basic operations like file input/output, networking, and drivers. Second, there's the Linux kernel with all the software packages like web browsers, e-mail clients, and office software that make for a good user experience. This is called a "distribution of Linux" and dozens of them exist. Distributions differ from each other by how they install, how they organize their files, what applications they come with, and what they look and feel like. Some distributions are produced by companies, some by students, some by organizations -- but they all work together, and development is a friendly rivalry.