SQL Databases are relational database management systems that use Structured Query Language (SQL) for defining, manipulating, and querying data stored in tables with predefined schemas and relationships.

Key characteristics include:

• ACID Properties: Atomicity, Consistency, Isolation, Durability for reliable transactions
• Structured Schema: Predefined table structures with defined data types
• Relationships: Foreign keys and joins to establish data connections
• SQL Language: Standardized query language for data operations
• Indexing: Performance optimization through database indexes

Popular SQL databases include MySQL, PostgreSQL, Microsoft SQL Server, Oracle Database, and SQLite. In AI/ML contexts, SQL databases are used for data storage, feature engineering, data preprocessing, and storing model metadata and results.