Peripherals are devices that can be added to a computer system [usually from the outside] and they can be grouped into one of two main groups:
Input devices
Output devices
Peripherals can be connected to the motherboard via ports that provide a way for the data going into or coming out of the motherboard to get there.
Common ports include:
AT
PSII
Com or serial
Parallel
USB 1, 2 and 3 (mini and micro)
Firewire
thunderbolt
esata