ARM is a family of processor architectures especially used in low-power devices. It is commonly found in many embedded applications in consumer and industrial products, in network appliances, in almost all smartphones and tablets, and in some servers.
ARM (the company) designs the instruction set and logical architecture. The physical architecture, as well as companion processors and boot code, is made by system-on-a-chip (SoC) vendors.
Available unix-like operating systems include linux ([tag:debian Debian, ubuntu [Ubuntu]2, arch-linux [Arch Linux]3, fedora Fedora, …), freebsd [FreeBSD]5, netbsd [NetBSD]6, openbsd [OpenBSD]7, … Most android and all ios devices have an ARM CPU as well.