In commodities futures trading, the Commitments of Traders (COT) report is the only official source of insight into the market positions of the key players. The COT reports give data on trillions of dollars of futures and options holdings in over 90 markets - everything from crude oil to the U.S. dollar, gold and the NASDAQ. The reports are issued every week by the U.S. Commodity Futures Trading Commission (CFTC). They can be downloaded for free from the CFTC's website each Friday at 3:30 p.m. (Eastern Standard Time). E.g. the COT report of Friday, Nov. 16, 2012 reflects a breakdown of Tuesday’s (Nov. 12) open interest for markets in which 20 or more traders hold positions equal to or above the reporting levels established by the CFTC (HERE - save as deacom.csv and open in MS-Excel). The CFTC classifies traders into three groups: 

(1) First are the commodity producers. Analysts call them the "smart money." Also known as commercial hedgers or just "the commercials," they are seen as the folks with the best market information. When they are increasing their positions in crude oil, for example, it's probably a good time to get some energy stocks. These traders are involved in the day-to-day operations of each commodity. They have an excellent handle on the underlying market, and it typically pays to follow their positions when they reach an extreme.

(2) Then there is the dumb money - the "non-commercial" category. These are the large speculative investment funds and large hedge funds, usually called the "large speculators" or simply the "large specs" - or just the "dumb money." They are primarily trend-followers, and will accumulate positions as a trend progresses. Analysts say they are usually wrong at market extremes. If they are buying oil, it's probably a good time to sell your energy stocks. When their positions reach an extreme, watch for a price reversal in the opposite direction of the existing trend.

(3) The third group is the "non-reportable" category. These are smaller traders, composed mostly of hedge funds and individual traders. These guys are seen as the really dumb money. They are the small-time traders who apparently don't know what they're doing at all and don't provide any meaningful market information. Or so say most analysts.

See the Commitments of Traders Explanatory Notes
Many speculative traders use the Commitments of Traders report to help them decide whether or not to take a long or short position. A conventional wisdom has it that the commercial hedgers and non-commercials (or large speculators) are consistently right, while the small speculators are consistently wrong. So one theory is that "small speculators" are generally wrong and that the best position is contrary to the net non-reportable position. Another theory is that commercial traders understand their market the best and taking their position has a better chance of profit (which is pretty much the same thing as the "small speculators" being wrong). However, Jeffrey A. Hirsch recommends the following: 

Keep in mind the commercials are sometimes not right. They are not in the market to time market turns; they are hedging their risk exposure in a cash position. Therefore the non-commercials, professional speculators, and funds, in the short term, are considered the "smart money." Here are some general guidelines to follow:
  • If non-commercials are net long, commercials are net long, and the non-reportable positions category are net short by at least a two to one margin, look at buying opportunities. In other words go long with the pros.
  • If non-commercials are net short, commercials are net short, and the non-reportable positions category are net long by at least two to one margin, look at selling opportunities.
  • If non-commercials are net long, commercials are net short, and the non-reportable positions category arc neutral, meaning not heavily net long or short, look at buying opportunities, and stick with the non-commercial "smart money."
In fact COT data has changed dramatically over the various periods in how it interacts with market prices, and there is no simple 101 to the matter. If anything, the commercial hedgers are now trading much more in lockstep with the S&P 500. But the relationship seems to evolve quite often, so using it alone to trade is probably very much a random exercise.  

To make sense of the weekly COT-data it is highly recommended to read some profound introductions to COT (e.g. of Lan Turner or Nikita Kabanovs), and to study Alex Roslin’s approach.

An interview with long time successful investor & trader Larry Williams in October 2005 on how to use COT reports, available HERE in mp3 format. And HERE another view on COT, specific to gold and silver markets.