From a practical perspective, schemas structure documents.

For example, a database is one form of document; a database schema describes:

• The specific properties of each field and table in the database
• The nature of the data that should be entered into each table or field
• The manner in which this data should be formatted.

As a practical example, a database schema might determine whether or not dates should be entered in a given field according to the MM-DD-YYYY or YYYY-MM-DD formats.

Database schemas thereby dictate where and how data should be entered into a database.

Schema validation is the process of checking data in a database against a schema. A database record containing data that has not been entered in accordance with the appropriate schema, such as data that has not been entered into the appropriate column or formatted in the appropriate way, is invalid.