The Raspberry Pi is a series of credit card-sized single-board computers developed in the UK by the Raspberry Pi Foundation with the intention of promoting the teaching of basic computer science in schools.
The original Raspberry Pi is based on the Broadcom BCM2835 system on a chip (SoC), which includes an ARM1176JZF-S 700 MHz processor, VideoCore IV GPU, and was originally shipped with 256 megabytes of RAM, later upgraded (models B and B+) to 512 MB. The Raspberry Pi 2 on the other hand, ships with a Quad-Core Cortex-A7 and a VideCore IV dual-core GPU and 1 GB of RAM.
Basically, a SFF(small form-factor) computer that can accomplish basic stuff, or if you are the tinkerer, you can create virtually anything, as the PCB provides a 40-Pin GPIO connector that can be connected to any sensor, motor or any component that uses a couple of pins for IO transfer.
Raspberry PI 2 includes 4 USB 2.0 , an RJ45 Ethernet port, an HDMI out, a DSI port to connect a small screen directly to the computing unit and a CSI port to attach a camera to the Computer. The power is given by a Micro-USB 2.0 that works with any phone charger, as it only needs 1A to work at basic functionality. Also, it is equipped with a 3.5mm Composite (Audio Video) out.
The projects realized with Raspberry Pi are very vast. They range from basic Media Center, to robots, hand-held retro gaming devices, Time-lapse Dolly, home-servers, surveillance systems and many more.
So, if you are into robotics or you want to create a simple machine for yourself, or even to make a little bit of money, than Raspberry Pi is for you, with all the expandability it comes with.