What is the Difference Between Similar But Different Things, Terms, and Objects

What is the Difference between Alertpay Authorize.Net PayPal Google Checkout

Internet has become a leading business opportunity thanks to several options in terms of efficient and economical payment modes. Alertpay, Authorize.Net, PayPal and Google Checkout are the names of such payment sources that work online and are really very secure ways of money transfer from one place to another either via bank account or email account.