In a relational database, a Primary key is used to uniquely identify each record in a table. This ensures that there are no duplicate records for any given entity and that each can be uniquely accessed. A Field/column is a different concept referring to an individual piece of data in a record, such as an employee's name or address. A schema defines the overall structure of the database, including tables and their relationships, but it itself does not uniquely identify records. A foreign key is a field in one table that uniquely identifies a row of another table, but it does not necessarily provide uniqueness within its own table.