Raspberry Pi is a small credit card sized computer, It is very cheap compared to the traditional computers, the Raspberry Pi Foundation has arrangements with a few manufacturers from whom you can buy a Pi directly at $25 - $35. The Raspberry Pi unlike the traditional computer, it has the ability to integrate with electronics projects. Raspberry Pi to control devices that a microcontroller can control like LEDs, relays, LCDs etc. A Raspberry Pi is better used where more processing power and connectivity is needed like for exemple when you want to remotely access a temperature sensor via the web to change its settings and download temperature log files and more in general Internet of Things (IoT) and Home Automation applications you should consider using the Raspberry Pi.