A PIN is a diode that has an additional layer included when compared to a traditional diode.  A PN diode has a p-doped region directly attached to an n-doped region creating a two-layer device.  A PIN diode includes an intrinsic layer in between the doped layers.  Intrinsic signifies a semiconductor that has not been doped with any dopant and has an equivalent value of holes and electrons.  The PIN diode p and n regions are also more heavily doped than normal to provide higher charge carrier capacity through the intrinsic area of the device.  Because of the additional layer, the capacitance is quite low in a reverse bias setup of a PIN.

PIN diodes are often used at high frequencies because that is the point they diverge from the operation of a typical diode.  When a frequency is low their operation is nearly identical, but as it gets higher the PIN diode begins to perform with resistive properties that can vary according to the frequency at a time, creating a resistor with a variable ohmage resistance.  This happens because charge can be stored in the intrinsic semiconductor, and it cannot completely drain when the frequency reaches a certain level.  Because of this, they are very common in RF and microwave circuits.