What is the difference between Primary Key and Foreign Key?

CREATE TABLE users (
  id INT PRIMARY KEY AUTO_INCREMENT,
  email VARCHAR(255) UNIQUE,
  name VARCHAR(100)
);

-- Composite primary key
CREATE TABLE order_items (
  order_id INT,
  product_id INT,
  quantity INT,
  PRIMARY KEY (order_id, product_id)
);

CREATE TABLE orders (
  id INT PRIMARY KEY AUTO_INCREMENT,
  user_id INT,
  total DECIMAL(10,2),
  FOREIGN KEY (user_id) REFERENCES users(id)
);

-- With actions
CREATE TABLE orders (
  id INT PRIMARY KEY,
  user_id INT,
  FOREIGN KEY (user_id) REFERENCES users(id)
    ON DELETE CASCADE
    ON UPDATE CASCADE
);