There are two giant upgrades in the Pi 3. The first is a next generation Quad Core Broadcom BCM2837 64-bit ARMv8 processor, making the processor speed increase from 900 MHz on the Pi 2 to up to 1.2GHz on the Pi 3.
The second giant upgrade (and this is the one we’re personally most excited about) is the addition of a BCM43143 WiFi chip BUILT-IN to your Raspberry Pi. No more pesky WiFi adapters – this Pi is WiFi ready. There’s also Bluetooth Low Energy (BLE) on board making the Pi an excellent IoT solution (BLE support is still in the works, software-wise)
Lastly, there’s an upgraded switched power source that goes up to 2.5 Amps instead of just 2 Amps – allowing your Pi to power even more powerful devices over USB ports.
The best part about all this is that the Pi 3 keeps the same shape, connectors, and mounting holes as the Pi 2. The only difference is a slight change in where the LEDs are. 99% of cases and accessories will still be fully compatible with the Pi 3, Pi 2, and Pi B + – though if you have a case with a metal top there might be some WiFi chip difficulties.