In hopefully simple terms there are three types of pilots licence...
The Private Pilots Licence, The Commercial Pilots Licence and the Air Transport Pilots Licence.
The licence dictates what you can do (in broad terms) when you fly. E.G. PPL allows flying for pleasure, or in the persuit of your business interests. A CPL allows you to charge for you services as a pilot (instructing, survey work, small charter etc) and ATPL allows you to command large commercial aircraft.
To any of these licences, you attach a rating - usually SEP (Single Engine Piston), MEP (Multi Engine Piston) or a Specific Type Rating. Usually you require a Type Rating for a Jet (or JetProp) aircaft, although there are type ratings for large piston engine. These ratings may be attached to any licence type. If you have an SEP or MEP you are allowed to fly any aircraft in that class provided you have the correct DIFFERENCES trainig to allow you to fly that particular aircraft (tailwheel, retract, VP etc). MEP types require you to be trained on type.
That covers what you can fly....where you can fly it depends upon the rules and the conditions in which you fly or find yourself. There are two rule set...visual and instrument.
To fly VFR you must maintain Visual Meterological Conditions (visibility and distance from cloud) for the class of airspace that you find yourself in. You cannot fly VFR in IMC - it's just not possible. You can, however, fly under IFR in either VMC or IMC. To fly in IMC you must be suitably qualified (IMCR or IR) and current.
As for flying a 747 under VFR - why ever not? You are subject to the same 'see and avoid' philosophy as everybody else - and being that big you're more likely to be seen and avoided. No rule against it. Indeed NJE often re-position VFR outside controlled airspace.