Most basic eCommerce stores use a "cart" to collect items/services and a "checkout" to collect payment and customer information. At the heart of Drupal Commerce is the ubiquitous Order. This Order is first created when a user adds a product to a cart. The Order is actually the "cart" when it has a status of "Shopping Cart." Since Drupal Commerce is an open eCommerce framework, these statuses can be added with additional modules (like shipping or customer profiles).
Below are some topics related to the add to cart form.