Nova is the second largest transaction processor in the United States. Basically Nova (platform) is the intermediary between the electronic payment gateway and the card associations. They are a stable / reliable company (so far) (they probably will be going through some changes soon and be consolidating more of their data centers).
Authorize.net is an electronic payment gateway and usually not available to people outside the United States. It is possible but usually takes a bit longer - and you need a merchant account provider to offer you this gateway and these providers usually only accept United States citizens.