There are certain features every computer needs in order to work. First, computers need power. The very basis of computing is in channeling electrons to flow through circuits. We rely on power cables and batteries for normal PCs. But a mini PC may not have an onboard battery or a place to plug in a power cord. Instead, it may draw power through a USB connection. The USB interface allows for the transfer of data and power. If the mini PC is in the form of a USB stick, plugging the computer into a powered display or USB hub could provide the power the computer requires to operate.
A computer needs a processor. The processor's job is to take data and to perform operations upon data to get a result. That result could be anything from displaying an image on a screen to simulating complex physics. Modern processors can have multiple cores, meaning the processor can work on more than one set of operations at a time. With certain types of computer problems, this decreases processing times. Many mini PCs rely on advanced reduced-instruction-set computer microprocessor (ARM)-based processors -- which tend to be small and energy efficient, giving off less heat than more powerful processors.
A computer also needs memory to store data. The processor can call upon data stored within memory and perform operations on it. There are two major categories of memory. Read-only memory (ROM) is unalterable and nonvolatile. That means you can't change what's stored in ROM and the information doesn't go away even if the computer loses power. The ROM in a computer typically stores system-level programs like basic input/output system (BIOS), which provides the set of instructions a computer needs to boot.
The other type of memory a computer relies upon is called random-access memory (RAM). A computer's RAM stores data by applying small electric charges to a series of memory cells. The information within RAM only exists as long as the processor needs it -- RAM can be repurposed according to the needs of the processor.
The mini PC also needs some storage medium that can hold information like the computer's operating system. Flash memory -- nonvolatile memory that comes in the form of an integrated circuit -- takes up little space and has no moving parts.
To do more than a narrow set of tasks, a computer needs an operating system. The operating system's job is to act as a platform for other programs and to allocate the computer's physical resources to those programs.
Finally, the PC needs some sort of physical interface that allows you to connect it to other devices such as displays, keyboards and other peripherals. Some mini PCs rely on USB connections. Others may incorporate standards like HDMI. Through these ports, the computer can communicate with other devices. Some have multiple ports -- one version of the Raspberry Pi computer has two USB ports, an Ethernet port, RCA-video out, an audio jack and HDMI port.