An intelligent-merge, relatively new DistributedVersionControl system. Arch imposes more structure on projects than do ConcurrentVersionsSystem or SubVersion; and the idea of distributed, peer-to-peer repositories with intelligent merges between them makes it a good choice for an OpenSource project.
Many arch proponents feel it is fighting for MindShare? with SubVersion. Where SubVersion fixes only the worst nuisances of CVS, arch goes further in the way of making a general, scalable configuration management system.
A useful piece of information is that arch comes in two implementations, namely larch and tla, of which tla is the newer and better supported one. So, if you are searching for the software, better search for "tla" than "arch". (Arch as a name is both very SearchEngineResistant and text-search resistant.)