What is a Primary Key and Foreign Key in MySQL?

Primary Key: Uniquely identifies each record in a table, cannot be NULL
Foreign Key: Links two tables together, references primary key of another table

Example:
CREATE TABLE users (
  id INT PRIMARY KEY AUTO_INCREMENT,
  name VARCHAR(100)
);

CREATE TABLE orders (
  id INT PRIMARY KEY,
  user_id INT,
  FOREIGN KEY (user_id) REFERENCES users(id)
);