What is a conventional tractor trailer truck?

A 'conventional' tractor is what is known to the rest of the world as either a 'bonneted' or sometimes 'Arctic' tractor. It simply means that the hood and engine compartment protrude in front of the cab, as opposed to a cab-over-engine tractor, where the engine compartment sits underneath the cab.

Currently, conventional tractors are all that's available new on the North American market - the last company to manufacture and market a cabover in North America was Freightliner with the Argosy, which has been discontinued for the US market, but is still manufactured for the South American, Australian, New Zealand, and southern African markets.