I get this question quite often, but I struggle explaining it, especially in a few simple words. Anyway, this is a very interesting topic because things are very complicated when it comes to UNIX vs Linux. There are business related things, licenses, policies, government influence etc.
Linux-based operating systems (often called Linux Distributions, or just Distros) are quite popular among programmers and developers since their announcement in the 90s. The Linux kernel itself is designed to be flexible and open for modifications and contributions, thus it can run on any hardware. The same principle is applied to almost the whole software stack above the kernel that constitutes the Linux Distribution as a complete product. In general, it is designed from programmers for programmers and freely available to everyone.