ARM is a RISC architecture designed for low-power and embedded use, e.g. smart phones and microcontrollers.
Summary from the ARM Wikipedia article:
ARM, originally Acorn Risc Machine, is a family of reduced instruction set computing (RISC) instruction set architectures for computer processors, configured for various environments, developed by British company ARM Holdings.
A RISC-based computer design approach means ARM processors require significantly fewer transistors than typical complex instruction set computing (CISC) x86 processors in most personal computers. This approach reduces costs, heat and power use. Such reductions are desirable traits for light, portable, battery-powered devices—including smartphones, laptops, tablet and notepad computers, and other embedded systems. A simpler design facilitates more efficient multi-core CPUs and higher core counts at lower cost, providing improved energy efficiency for servers.