There are three main points Karl Marx describes in "The Commodity", which is Chapter One of "Das Kapital". He describes basic principles of value and exchange after defining the term commodity. Marx uses the term commodity to mean an external object which satisfies either directly or indirectly a human need. The commodity can be a literal object such as corn or iron. It can also be an abstract item such as art which satisfies the human condition.
Marx introduces the idea of value through an examination of three types of value. The first is the "use-value" of an object. This is how useful the commodity is to society. A commodity by itself has no inherent use-value, unless someone wants it. Therefore, the use-value of a commodity is tied directly to its demand. The use-value of a commodity will differ among people, but society will eventually decide a generic use-value based on demand prevalence.
Use-value is also connected to "exchange-value", which is the amount of one commodity needed to trade for another commodity. In theory, a certain amount of corn will equal an amount of iron in an equal trade. This is the exchange value. It will also fluctuate according to demand and the use-value of each commodity. In order to properly establish the exchange-value, each commodity must share some other aspect so a fair equation can be calculated. This aspect is the third point and is described as "value". Value, according to Marx, represents the effort to produce the commodity. Value will increase as effort to produce increases. The inverse is also true. Value is not a set aspect of production but depends on demand. A commodity without demand is useless and therefore it has no value.