The Raspberry Pi 3 is a credit-card sized computer which is capable of many of the things that your desktop PC does, like spreadsheets, word-processing and playing high-definition video and games. It can run Linux (and even Windows 10 free-of-charge) and is being used to teach kids all over the world how to program.
The Raspberry Pi 3 computer is so small and powerful as it uses the Broadcom BCM2837, an ARM Cortex-A53 64bit Quad Core Processor System-on-Chip. The GPU provides Open GL ES 2.0, hardware-accelerated OpenVG, and 1080p30 H.264 high-profile decode and is capable of 1Gpixel/s, 1.5Gtexel/s or 24 GFLOPs of general purpose compute. What this means is that if you plug the Raspberry Pi 3 into your HDTV, you could watch BluRay quality video, using H.264 at 40MBits/s.
Additionally, the Raspberry Pi 3 has improved power management, with an upgraded switched power source up to 2.5 Amps, to support more powerful external USB devices.
On top of all that, the low-level peripherals on the Pi make it great for hardware hacking. The 0.1" spaced 40-pin GPIO header on the Pi gives you access to 27 GPIO, UART, I2C, SPI as well as 3.3 and 5V sources. Each pin on the GPIO header is identical to its predecessor the Model B+.