From RaspberryPi.org:

"The Raspberry Pi is a low cost, credit-card sized computer that plugs into a computer monitor or TV, and uses a standard keyboard and mouse. It is a capable little device that enables people of all ages to explore computing, and to learn how to program in languages like Scratch and Python. It’s capable of doing everything you’d expect a desktop computer to do, from browsing the internet and playing high-definition video, to making spreadsheets, word-processing, and playing games."

There are multiple versions of the Raspberry Pi:

• Raspberry Pi A +
• Raspberry Pi 2 Model B
• Raspberry Pi Model B +
• Raspberry Pi 3
• Raspberry Pi 3 Model B+
• Compute Module
• Compute Module 3
• Compute Module 3 Lite

There are a lot of projects built with the Raspberry Pi. If you are looking for ideas there are a 20 ideas here.