With parallel the flow rate is shared between the components and the overall restriction is halved (assuming that the components are the same design). One potential issue is if you run a high restriction and a low restriction component in parallel most of the flow will go through the low restriction component. This creates problems with flow balancing.
Some people run their GPU blocks in parallel to reduce the overall loop restriction. Parallel GPU blocks improves the flow rate through other components like the CPU block. As GPU blocks are not all that sensitive to flow rate compared to CPU blocks it is a trade off between a slight rise in GPU temperature compared to a larger drop in CPU temperature.Series:
With serial the flow rate is the same through all components and the overall restrition is summed. Serial removes any issues that might arise with flow balancing and makes things simpler. Most people plumb their loop in series.
With pumps it is important to plumb them in series. The flow rate of a loop is determined by how hard it is to push water through components, something usually referred to as restriction. Restriction is overcome by pressure - the higher the pressure at the pump outlet the faster the water can flow through the loop.
Under normal operation series provides higher pressure which overcomes the restriction to provide higher flow rates.. Putting pumps in series does not provide higher flow rates as the pressure stays roughly the same.
If one pump were to fail in series then the water will continue to flow normally, albeit with reduced flow rates. In parallel the water will recirculate through the stopped pump, dramatically reducing the loop flow rate. Because of this pumps in series can be said to double your reliability whereas parallel halves it.Edited by GingerJohn - 8/28/12 at 1:59pm