Data models specify and declare how data is to be structured. The model can be formal — as with that declared in SQL — or it can be an informal document describing it.
Formal or Informal
The level of formality of the data model depends on the scope, importance, and usage of the data it models. A shopping list follows a data model. So does a government database of registered voters. Determining the level of formality needed — and specifying how it will be captured and communicated — is the very essence and art of data architecture.