When the original Raspberry Pi came out, it could only run Raspbian on it. Raspbian is a free operating system based on Debian optimized for the Raspberry Pi hardware. Then the Raspberry 2 came out, a bit more powerful with a quad-core Broadcom BCM2836 ARM v7 processor at 900MHz. Now, you can run Ubuntu Mate on it. Ubuntu has a project dedicated for Raspberry Pi 2. It’s still not powerful to run Ubuntu Unity desktop, but Ubuntu Mate works quite well on the Raspberry Pi 2.