SQL (Structured Query Language) is a standard language used to manage and manipulate relational databases. It is used to create, read, update, and delete (CRUD) data, as well as to manage database structure and permissions.

A relational database organizes data into tables, which consist of rows and columns. Relationships between tables are established through primary and foreign keys, which enable data to be accessed in related tables.

A primary key is a column (or a combination of columns) that uniquely identifies each row in a table. A table can have only one primary key, and the values in this column(s) must be unique and not NULL.

A foreign key is a column or a set of columns in one table that references the primary key of another table. It is used to establish and enforce a link between the data in two tables, ensuring referential integrity.