Through tables, and the relations between them which keeps the data consistent. The structure of the data is mostly put down to the gathered information for example; If I had a business that handled credit/debit card details and I wanted to structure the data into a database. I wouldn't stick all the information on one table, instead I would structure it out so that I would have different tables specific to genre. I.E a table for personal details and then another table with the card details. Between the two I would need to set-up a relationship so that the data is consistent and actually has an efficient purpose. Look up normalisation for database's you'll find a lot of information about the structured data in relation to a database.