VFR - Visual Flight Rules
Flying and navigating the aircraft, by reference to the natural horizon and features on the ground or water.
NVFR - Night Visual Flight Rules
Flying the aircraft, by visual reference however at night... However at night because you can't see the horizon as well or features on the ground or water, especially if its a moonless night... about 50%- 80% is done on instruments, depending on where you are.
To fly VFR or NVFR a pilot must maintain what is known as Visual Meteorological Conditions (VMC) the pilot must maintain a certain visibility and vertical and horizontal distance from cloud at all times.. Otherwise s/he would be breaking the law
IFR - Instrument Flight Rules
Flying and navigating the aircraft by sole reference to the instruments only.. Only the take off and landing are done visually,
Its mainly for bad weather, where visibility is reduced and you cant see the horizon to maintain the aircraft attitude and ground features to navigate..
Keep is mind, you may also fly IFR even if the weather is good... It is just a set of rules pilots follow..
To be able to fly IFR, you also need an Instrument rating.. Because flying under instruments requires a fair bit of skill
... VFR is the default form of flying when a pilot is issued a licence..