A modular phone is a new smartphone design in which different pieces can be swapped out for different kinds of functionality. Developer collaborations like Project Ara are working on prototypes for this type of phone.
Modular phones are also known as mobile transformers.
The idea behind modular phones is that users can customize their phones according to their own needs. For example, modules for cameras of different resolutions, or different size memory modules can easily be swapped out when needed. In this model, when a user needs a particular feature on their phone, they can simply install a new module for it, rather than having to buy an entirely new phone.
In general, Google's Advanced Technology and Projects (ATAP) group, which, according to some experts, emerged from DARPA, is behind the development of the modular phone and provides specifications for how this type of phone would work.
Developers working on modular phones are looking at the internal technology, for example, the passive components that will provide the different kinds of functionality. At the same time, designers are also looking at the aesthetic aspect of the phone so they could design the modules well, so as not to make the phones look overly clunky.
Modular phones are still under development, but some are being presented at various technology websites as they evolve.