Linux is an operating system that was initially created as a hobby by a young student, Linus Torvalds, at the University of Helsinki in Finland. Linux is similar but not identical to UNIX. The kernel, at the heart of all Linux systems, is developed and released under the GNU General Public License and its source code is freely available to everyone. It is this kernel that forms the base around which a Linux operating system is developed. There are now probably hundreds organisations that have released their own versions of operating systems based on the Linux kernel. Some common examples of Linux distributions are RedHat, SuSe and Debian.
As an aside, the kernel is the heart of the linux operating system. It controls access to files, allocates resources, maintains the file system, manages the memory, and much more. Most users do not have to deal in-depth with the linux kernel.
Apart from the fact that it's freely distributed, Linux's functionality, adaptability and robustness, has made it the main alternative for proprietary Unix and Microsoft operating systems. IBM, Hewlett-Packard and other giants of the computing world have embraced Linux and support its ongoing development.
Linux is unambiguously the operating system of choice for scientific computing. There exists a wide variety of high level languages, debugging tools and other code development tools for programming, numerical subroutines for solving various types of equations, plotting and visualization packages, and parallel programming software. If you want to learn more about scientific computing, you need to learn how to use linux.
The terminology used in linux might be a little different than what you're used to. For example, what you might call a Folder is called a Directory in linux.