Single-board computers (SBCs) are complete computers built on a single circuit board with microprocessors, memory, input/output, and other features. They are typically used as embedded systems, as they are small, low-cost and low-power. SBCs are used for a variety of applications, a few examples being robotics, industrial control and home automation. They are also commonly used in education, as they are easy to use and can provide a great introduction to programming and computer science.
Some examples of SBCs include:
Each of these boards has its own unique features but they're all fairly powerful, low-cost and low-power, making them ideal for many applications. Currently the Raspberry Pi being the most popular and widely used.