export const metadata = {
  title: `Cart Concepts`,
}

# {metadata.title}

In this document, you’ll learn about the main concepts related to carts in Medusa.

## Cart

A cart is the selection of product variants that a customer intends to purchase. It is represented by the [Cart data model](/references/cart/models/Cart).

A cart holds information about:

- The items the customer wants to buy.
- The customer's shipping and billing addresses.
- The shipping methods used to fulfill the items after purchase.
- The payment method and information necessary to complete the purchase.
    - These are stored and handled by the [Payment Module](../../payment/page.mdx).

---

## Line Items

A line item, represented by the [LineItem](/references/cart/models/LineItem) data model, is a quantity of a product variant added to the cart. A cart has multiple line items.

<Note>

In the Medusa application, a product variant is implemented in the [Product Module](../../product/page.mdx).

</Note>

A line item stores some of the product variant’s properties, such as the `product_title` and `product_description`. It also stores data related to the item’s quantity and price.

---

## Shipping and Billing Addresses

A cart has a shipping and billing address. Both of these addresses are represented by the [Address data model](/references/cart/models/Address).

![A diagram showcasing the relation between the Cart and Address data models](https://res.cloudinary.com/dza7lstvk/image/upload/v1711532392/Medusa%20Resources/cart-addresses_ls6qmv.jpg)

---

## Shipping Methods

A shipping method, represented by the [ShippingMethod data model](/references/cart/models/ShippingMethod), is used to fulfill the items in the cart after the order is placed. A cart can have more than one shipping method.

In the Medusa application, the shipping method is created from a shipping option, available through the [Fulfillment Module](../../fulfillment/page.mdx). Its ID is stored in the `shipping_option_id` property of the method.

### data Property

After an order is placed, you can use a third-party fulfillment provider to fulfill its shipments.

If the fulfillment provider requires additional custom data to be passed along from the checkout process, set this data in the `ShippingMethod`'s `data` property.

The `data` property is an object used to store custom data relevant later for fulfillment.