Google Now is a feature of the Google Search app for Android that shows information about what's happening right now or in the near future: weather, calendar events, reservations, travel information. Try this: disable Google Now in Google's search app and you can still use voice search. After all, Google Voice Search has been available before Google Now. Google Search for iOS includes Voice Search, but it doesn't have Google Now yet.
One of the explanations why many people confuse Google Now with Google Voice Search is that Google Now and the improved Voice Search have been announced at the same time, when Jelly Bean was released. Google Now is shorter and sounds better than Google Voice Search.
Even the Wikipedia article for Google Now is inaccurate: "Google Now is an intelligent personal assistant available for Google's Android operating system. An extension of Android's native Google Search application, Google Now uses a natural language user interface to answer questions, make recommendations, and perform actions by delegating requests to a set of web services. Along with answering user-initiated queries, Google Now passively delivers information to the user that it predicts they will want, based on their search habits."