Surge Protection Installation
Surge Diverters (SPD in parallel,left image) are protection devices typically connected in parallel to the load. Metal Oxide, Varistors,Gas Arrestors and Spark Gaps, etc. are used in these devices. They provide only shunt diversion and are commonly used at the point of entry to a facility. The offer only coarse protection. 

In other words: Only the voltage is diverted but a surge current to the load is not controlled.

The picture shows a 800 kV surge diverter (Siemens)

 
Surge filters (SPD in series, right image) are typically secondary series connected devices that combine shunt and series elements to provide fine protection against surges. You find them mostly further downstream, closer to the load. They contain the same technology elements as the surge diverters but also utilize a combination of inductors and capacitors. The addition of these 2 additional  elements add the ability to control or block also the surge current.

In other words: They divert the voltage and control current.