I need help understanding this sql code/script, but using oracle SQL Developer:

CREATE TABLE Product (

ProductId INT PRIMARY KEY,

ProductDescription VARCHAR(200)

);

CREATE TABLE Item (

ItemNum INT PRIMARY KEY,

ItemDescription VARCHAR(200)

);

CREATE TABLE Receipt (

ReceiptNumber INT PRIMARY KEY,

SalesDate DATE

);

CREATE TABLE Sales (

ReceiptNumber INT,

ProductId INT,

QuantitySold INT,

PRIMARY KEY (ReceiptNumber, ProductId),

FOREIGN KEY (ReceiptNumber) REFERENCES Receipt(ReceiptNumber),

FOREIGN KEY (ProductId) REFERENCES Product(ProductId)

);

CREATE TABLE Usage (

ReceiptNumber INT,

ProductId INT,

ItemNum INT,

QuantityUsed INT,

PRIMARY KEY (ReceiptNumber, ProductId, ItemNum),

FOREIGN KEY (ReceiptNumber) REFERENCES Receipt(ReceiptNumber),

FOREIGN KEY (ProductId) REFERENCES Product(ProductId),

FOREIGN KEY (ItemNum) REFERENCES Item(ItemNum)

);

Also i would like an explanation as to the differences in order for it to work.