Microcontrollers are essentially entire computers contained on one single chip. Because of their very nature, they are capable of performing very complex tasks, of being programmed and of functioning exactly as a computer does, only on a much smaller scale. They are utilized heavily in industry for a variety of applications, as well as in consumer devices.
For education and development purposes, microcontroller-based projects and development kits are oftentimes sold as bundles, containing the basics of what someone would need to put together a specific project. In the case of development kits, the offerings may be more broad, owing to the fact that the end user may use the kit for any of an array of different purposes. A project, on the other hand, is generally something that is sold as a product that the user puts together, with building the project being part of the overall user experience.