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SQL (Structured Query Language) is a standard programming language specifically designed for managing and manipulating relational databases. It is used to query, insert, update, and delete data, as well as to manage database schema and control access to the database.

SQL commands can be broadly categorized into the following types:

• DDL (Data Definition Language): Commands like CREATE, ALTER, DROP, which define the structure of the database.
• DML (Data Manipulation Language): Commands like SELECT, INSERT, UPDATE, DELETE, which are used for data manipulation.
• DCL (Data Control Language): Commands like GRANT, REVOKE, which control access to the data.
• TCL (Transaction Control Language): Commands like COMMIT, ROLLBACK, which manage transactions in the database.

A primary key is a field in a table which uniquely identifies each row/record in that table. Primary keys must contain unique values, and cannot contain NULL values. A table can have only one primary key, which can consist of single or multiple columns (fields).