PRIMARY KEY in SQL is a constraint that uniquely identifies each record in a database table. It ensures that no duplicate values exist in the primary key column(s) and none are NULL. A table can have only one primary key, consisting of a single or multiple columns (composite key). The primary key is crucial for establishing relationships between tables and ensuring data integrity. It also automatically creates an index on the column(s), enhancing query performance. PRIMARY KEY constraints are defined during table creation or added later using the `ALTER TABLE` statement.