Thats a great question. Alot goes into reciving the ability to proccess credit cards for a business. Its a nessisary evil and some merchants have to think about it more than others. Business are broken down to three catagories, A, B and C class business. A typical class A type business is your local pizza place, doctors office or any retail establishment where there is a a face to face transaction and the customer recives their good or service at that time.
Class B business are like your major retailers with an online pressence. A business that has an online store but an actual retail outlet where a customer make make an instore return or exchange of goods.
Class C merchants are any online business, mail order or drop shipping business with 100% future delivery of a good or service and no physcial retail outlet. In order to recive a competative proccessing rate through and actual bank proccessor stiff criteria have to be met.
In recient years GOOGLE and PAYPAL have become solid alternatives for a small business to accept credit cards online. But, they are not with out there restrictions. Transactional limits, verification holds on funds to veryify the source, can all casuse huge headaches for a small business that is trying to really make money from their online transactions.
Plus not to mention the bible of all credit card transaction: The Rules For VISA Merchants.
http://www.usa.visa.com/download/merchants/rules_for_visa_merchants.pdf
I feel this is all very usefull information for a small business that really wants to get their business moving online.
Not to mention the basics like whats the diffrence between a shopping cart and a payment gateway.
Talking about these issues and the diffrences could be very benificial to a small business.