Hi MrMiyagi,

A shopping cart is for presenting the items/choices a visitor wants to purchase, this includes product number (SKU or ID), price, additional attribrutes (i.e. size, color, qty, etc.)

Merchant Account: is typically your business bank account. This is for transactions between the customers bank (i.e. credit card) and your bank.

There's also a third piece to the transaction process, it's called a "payment gateway". This sits between a customers bank and your bank to manage the money transaction. There's a number of payment gateways available; PayPal, Authorize.net, Cybersource, etc.

Your e-commerce website "glues" all of this together... product selection, order, payment, delivery, etc.

I hope this helps.