The data structure commonly used for implementing relational databases is the table. Tables store data in rows and columns, where each row represents a unique record, and each column corresponds to an attribute of the record. This structure allows for efficient data organization and retrieval using SQL (Structured Query Language). Furthermore, relational databases utilize relationships, such as primary and foreign keys, to connect data across different tables, enabling complex queries and ensuring data integrity. This structure has become foundational in managing structured data.