Where do you see two Google Search apps? Is it in your app drawer or in the app manager, under "Running"? If it's the running apps list, keep in mind that Android may list an app more than once, depending on which various processes or services related to the app are running.
Google Search is the overall app. Google Now is a function of Google Search, and is typically activated by holding the soft home button down until the Google semicircle appears, then swiping up. Google Now will give you a bunch of useful information that you can select in its settings (like local weather, sports scores, upcoming events drawn from your Calendar, etc.), and over time, it's supposed to learn what you really want to know. Here's a useful guide describing how to use it:
http://forums.androidcentral.com/ambassador-guides-tips-how-tos/300182-guide-google-now.html
Google Voice is only if you use the Google Voice service, which is a free service that gives you a phone number that you can associate with any of your existing phone numbers, and gives you some useful control over how you receive calls. You can also receive transcriptions of your voicemail. Learn more here:
Google Voice - Features