Quote from PARACLESE:
trading pairs = relative value. One manager/trader understands all companies within one sector are going from a-g while at trading at disparity with one another, historically speaking, they all tend to move back towards the mean. So, a good trader who rigorously follows his companies, will most likely be able to catch the top in the overvalued=sell, while buying the bottom in the undervalued or the laggard, to collect the interim spread disparity between the two companies within the same sector. This long/short strategy is labeled market neutral since you are long and short within the same sector, your market risk is supposed to be smaller. As mschey said, this could bleed you badly if you are not careful. If you have a smaller name which is making leaps and bounds and you decide to short that one, then you buy the company which is larger and lagging, the one you bought decides it will purchase the high flyer, the spread will widen -long/+short and you will most likely die. hope this helps the pair trading head scratchers.