The Raspberry Pi 3 is the third generation Raspberry Pi. It replaced the Raspberry Pi 2 Model B in February 2016.
Because it has an ARMv8 processor, it can run the full range of ARM GNU/Linux distributions, including Raspbian, Snappy Ubuntu Core as well as Microsoft Windows 10.
The Raspberry Pi 3 has an identical form factor to the previous Pi 2 (and Pi 1 Model B+) and has complete compatibility with Raspberry Pi 1 and 2.
The Raspberry Pi 2 is powered by the microUSB port so you'll need a power supply to go with it.