Flying clear of clouds
There are two sets of rules by which flights are conducted in most countries: visual flight rules (VFR) and instrument flight rules (IFR). If the weather is above certain established standards—roughly that the visibility is above the minimum for flying by visual reference alone—pilots may make flights by visual flight rules. While operating under VFR, a pilot may fly into and out of uncontrolled airports without any requirement to speak to a controller.
Just as in the real world, you don't have to interact with ATC in Flight Simulator when flying VFR. In fact, Flight Simulator is even more flexible, because you don't have to talk with controllers even in VFR situations that would require communication in the real world.
The point to remember when flying VFR in Flight Simulator is that ATC is there to help you. You can communicate when flying into airports, request transitions through controlled airspace, and request Flight Following service (see Airspace Transitions and Flight Following.)
Related Links: Introduction to Air Traffic Control IFR Flight and ATC Understanding Airspace ATC Glossary
Another advantage of talking with controllers while under VFR is that the controller will tell you about air traffic flying near you. The controller won't tell you about every aircraft, but will help keep you advised when the workload permits. (See Traffic Messages from ATC).
You can use the Flight Planner to plan a trip from one airport to another in VFR conditions (see Using the Flight Planner.) By using the Flight Planner, you can use the GPS to follow your route and even have the autopilot track it for you. You're not filing a flight plan when you use the Flight Planner under VFR, so there's no flight plan that you must cancel at the end of your flight (as is required in the real world).
Flying VFR in IFR Weather Conditions
You will be denied permission to take off or land if the weather at the airport is below VFR minimums. If the destination airport weather turns IFR after you're cleared to land, however, you'll be allowed to land.
If you want to mimic real-world airspace regulations, you should communicate with ATC while VFR when