A contract is just one or a group of promises to be executed by the contract law, while contract law is a legal system that defines the right of one or more doers to execute the promises of another doer.
Contract principles vary from market to market. In many markets, private doers choose to deviate from the rules provided by the state and formulate their own industry contract law guidelines. Contract laws in varies from jurisdiction to jurisdiction, especially between jurisdictions with common law and civil law traditions.
The differences in contract law between different markets are not necessarily small, and even the most basic contract principles in common law have deviations. There are fierce debates about whether the rules of contract law should change in different markets and to what extent.
One view is that the normative theory of contract law is generally applicable, which provides certain substantive rules of contract law for all markets. The opposite is a special theory that stipulates different rules of contract law for different markets, that is, different types of agreements should be interpreted differently.