What is the difference between SQL and NoSQL databases?

SQL databases are relational, using structured tables with predefined schemas, supporting ACID transactions and complex queries via SQL. NoSQL databases are non-relational with flexible schemas - types include document (MongoDB), key-value (Redis), wide-column (Cassandra), and graph (Neo4j). NoSQL offers horizontal scaling and schema flexibility but often sacrifices ACID for eventual consistency. Choose based on data structure, scalability needs, and consistency requirements.