Business::onlinepayment::authorizenet is a business::onlinepayment backend module for authorize.net. it is only useful if you have a merchant account with authorize.net: http://www.authorize.net/

business::onlinepayment is a generic interface for processing payments through online credit card processors, online check acceptance houses, etc. (if you like buzzwords, call it an "multiplatform ecommerce-enabling middleware solution").