Will it remember a certain area and I never have to put in the NAV disc while traveling within this area, or do I need the disc every time I want the system to guide me somewhere?

You can plot a route with the disk in, pull it out and it will still remember the route after you remove the disk

So every time a new route is created, I need the disc? Or what happens if I take a side-trip off my plotted route...will I need the disc for it to guide me back to the original route?

You will need the disk to plot a new route, do a POI search, etc. If you go off your route, it saves a certain area of the map around your route so that it can reroute back to the original route without the disk