A commodity is a good where one unit is interchangeable with another of the same specification. One barrel of a given grade, at a given place and time, is treated as equivalent to any other barrel meeting that spec. That interchangeability is what makes a market possible: buyers and sellers can agree a price without inspecting each unit.
Crude oil is a weaker version of that ideal than the word suggests. There are hundreds of distinct crude streams, and they are not interchangeable at all in a refinery: a light sweet grade and a heavy sour grade yield different products, need different processing units, and are worth materially different amounts. What the market does is pick a small number of reference grades, trade those to death, and price everything else as a differential to one of them. Fungibility is manufactured, not found.
Trade happens in two connected layers, the physical and paper markets, and it is populated by two kinds of participant, hedgers and speculators. What the market produces is not one price but a set of them, one per delivery month, published each day as settlement prices. Those prices, laid out against their delivery dates, are the forward curve.
Three consequences follow from that structure, and most of what a desk does day to day is one of them.
First, because there is a price for every delivery month, there is a price for time. The gap between two months is a spread, and it says what the market will pay to have a barrel sooner rather than later. Second, because the reference grades are few and the real grades are many, there is a price for quality and location, which is the differential market. Third, because crude is an input to something else, there is a price for the transformation, which is the crack spread.
None of those three is a forecast. They are all statements about relative value right now, and that distinction is the whole reason a desk can trade them at all: a view on the level of crude is a macro bet, while a view on a spread is a bet about a mechanism you can name.