A modular PC is a computer that has individually-housed components, which are interconnected but separately removable for service or upgrading.
The format means that users can replace most components themselves because replacing a component does not require them to open enclosures, such as PC cases, or touch ESD-sensitive circuit boards. Modularity also makes the computer more environmentally friendly because users are more likely to replace broken components rather than junking the whole PC and creating e-waste.
A number of companies have developed prototype or production modular PCs. Razer’s concept project Christine is a high-end modular desktop with easily interchangeable mineral oil-cooled component modules featuring no-leak quick disconnects.
The Ice Computer's xPC is a low-end PC that offers easy upgrading. With xtablet, the user can expand with the xKeypad for more serious typing, or the xAdapter with xPC module or xTop to expand processing ability. The Xi3 Piston is an extremely compact and power-efficient gaming computer. While Piston is upgradable, modules are not actually individually housed, making them more similar to the way typical desktops are modular, with bare PCB-based parts.
See a modular computer the size of your phone: