Small cells, originally, were defined as small cellular base stations. That definition has evolved over time with the emergence of Wi-Fi networks, and the term now encompasses a somewhat broader scope than just cellular. Essentially, all small cells serve the same purpose. As one goes up the food chain, each platform has enhanced capabilities, broader coverage, and more functionality.
For a time, small cells were, exclusively, deployed by mobile network operators (MNOs) to fill in RF voids, as best as possible, where placing macrocells was not practical or cost-effective. Now, small cells have moved to the consumer and commercial arena and, with the ubiquitous proliferation of Wi-Fi, they are used for a multitude of scenarios.
Small cells come in four basic models, femto, pico, micro and metro. Today, they can be licensed, or unlicensed. Essentially, the definition of each type of small cell is related to its coverage area, power output, and application.