CubeSats in a nutshell
A CubeSat is a type of very small satellite which is based on a standardized unit of mass and volume.The initial basic CubeSat unit measured 10x10x10 centimeter, conforming to specific interfaces for allowing a standardized containerized launch and had a maximum mass of 1 kilogram (the mass was later on increased to 1,33 kilogram). It was quickly realized that such basic CubeSat units could be combined to form slightly larger spacecraft while mostly adhering to the same requirements and constraints. Multiples of the basic CubeSat unit were combined together to establish larger CubeSats. For example, a 1-Unit CubeSat measures one single basic CubeSat unit as described above, while a 3-Unit CubeSat consists of 3 standard CubeSat units stacked together. The CubeSat concept has become very popular, both in university groups, as well as for researchers, space agencies, governments, and companies.CubeSats offer a fast and affordable way for a wide array of stakeholders to be active in space and allow for a fast innovation cycle.
Source: ISIS - Innovative Solutions In Space