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A structured database, also called a relational database, follows a strict, predefined schema that organizes data into tables that have rows and columns. Reliability is ensured with atomicity, consistency, isolation, and durability principles. Structured Query Language (SQL) is used to retrieve and manipulate data. Structured databases also support complex relationships between data using foreign keys and joins.

A NoSQL database is a non-relational database that stores and retrieves data without a defined schema. These databases are more flexible and optimized for scalability and better handle unstructured data. It prioritized availability and partition tolerance over strict consistency. They support a variety of data models like document databases, column stores, graph databases, and key-store values.

Structured databases scale vertically and NoSQL scales horizontally. What this means is in order to make a structured database faster, you need faster hardware. With NoSQL, the horizontal scaling means you can scale simply by adding more nodes.

Which you use all depends on the type of data you are storing, how it’s being queried, and what kind of load you expect on the database.

ACID (Atomicity, Consistency, Isolation, Durability) is a set of key principals that databases follow to ensure data integrity and reliability.

Atomicity

This ensures that a transaction is a single, indivisible unit. The transaction either completes fully or not at all. If any part of the transaction fails, the operation is rolled back for data integrity.

Consistency

This guarantees that a transaction will maintain integrity of the database by ensuring it transitions from one valid state to another. If a transaction doesn’t follow the database's rules, for example a 0 in a field that does not allow a 0 value, the transaction is denied instead of completing.

Isolation

Ensures that multiple simultaneous transactions do not affect each other. This prevents data conflicts, like assigning something multiple times that is meant to be unique. There are different isolation levels that have tradeoffs between data integrity and performance.

Durability

Chances must be permanent. Once a transaction is recorded, it should survive sudden system crashes or total system loss through power loss or other means. In short, don’t write only to memory. Techniques like write-ahead logging, replication, and checkpointing are used to ensure this requirement is met.