Raspbian (as well as NOOBS, a beginner-friendly compilation of several popular operating systems designed for Raspberry Pi). As Raspberry Pi 2 is built using the ARMv7 processor, the latest Raspbian now comes with a Linux kernel built for ARMv7: "Raspberry Pi 2 is available to buy today. Remember you'll need an updated NOOBS or Raspbian image including an ARMv7 kernel and modules from our downloads page. At launch, we are using the same ARMv6 Raspbian userland on both Raspberry Pi 1 and 2; over the next few months we will investigate whether we can obtain higher performance from regular ARMv7 Debian, or whether we can selectively replace a small number of libraries to get the best of both worlds. Now that we're using an ARMv7 core, we can also run Ubuntu.
Raspbian is a free operating system based on Debian GNU/Linux and optimised for the Raspberry Pi hardware (the armhf processor architecture). Raspbian comes with over 35,000 packages, or pre-compiled software bundled in a nice format for easy installation on a Raspberry Pi. The initial build was completed in June of 2012, but the distribution continues to be active developed with an emphasis on improving the stability and performance of as many Debian packages as possible. Although Debian produces a distribution for the arm architecture, it is compatible only with versions later than the one used on the Raspberry Pi (ARMv7-A CPUs and higher vs the Raspberry Pi's ARMv6 CPU).