Microcontrollers are built using a microcontroller case in the assembler. They have less functionality compared to computers, but are cheaper to build. In particular, they can not interact with external components.
Microcontrollers can take various components, such as CPUs, memory (RAM), and Expansion cards. Microcontrollers are unable to contain a hard disk drive, but do contain a slot for an EEPROM, which can be programmed for very specific tasks. They have an advantage over computer cases, in that they can also make use of some upgrades, such as the piston upgrade.