Where can I find the exact definition of VFR and IFR?
The Rules of the Air are defined by ICAO and made applicable with few deviations in the ICAO member states.
ICAO Annex 2 Rules of the Air, defines in addition to the general rules, the Visual Flight Rules (VFR) and the Instrument Flight Rules (IFR). Flights shall be in compliance with either of them.

2.2 Compliance with the rules of the air
The operation of an aircraft either in flight or on the movement area of an aerodrome shall be in
compliance with the general rules and, in addition, when in flight,
either with:
a) the visual flight rules; or
b) the instrument flight
rules.

Chapter 5 of Annex 2 contains the Instrument Flight rules.
Chapter 4 of Annex 2 contains the Visual Flight Rules.

4.1 Except when operating as a special VFR flight, VFR flights shall be conducted so that the aircraft is flown in conditions of visibility and distance from clouds equal to or greater than those specified in Table 3-1.

Table 3-1 is summarized below.
+---------------------------+-------------------+---------------------+-----------------------------+
|      Altitude band        |   Airspace class  |  Flight visibility  |     Distance from cloud     |
+---------------------------+-------------------+---------------------+-----------------------------+
| At and above 3 050 m      | A*** B C D E F G  |        8 km         | 1 500 m horizontally        |
| (10 000 ft) AMSL          |                   |                     | 300 m (1 000 ft) vertically |
+---------------------------+-------------------+---------------------+-----------------------------+
| Below 3 050 m (10 000 ft) | A*** B C D E F G  |        5 km         | 1 500 m horizontally        |
| AMSL and above 900 m      |                   |                     | 300 m (1 000 ft) vertically |
| (3 000 ft) AMSL, or above |                   |                     |                             |
| 300 m (1 000 ft) above    |                   |                     |                             |
| terrain, whichever is the |                   |                     |                             |
| higher                    |                   |                     |                             |
+---------------------------+-------------------+---------------------+-----------------------------+
| At and below 900 m        |   A*** B C D E    |        5 km         | 1 500 m horizontally        |
| (3 000 ft) AMSL, or 300 m |                   |                     | 300 m (1 000 ft) vertically |
| (1 000 ft) above terrain, +-------------------+---------------------+-----------------------------+
| whichever is the higher   |        F G        |        5 km**       | Clear of cloud and with the |
|                           |                   |                     | surface in sight            |
+---------------------------+-------------------+---------------------+-----------------------------+
* When the height of the transition altitude is lower than 3 050 m (10 000 ft) AMSL, FL 100 should be
  used in lieu of 10 000 ft.
** When so prescribed by the appropriate ATS authority:
    a) flight visibilities reduced to not less than 1 500 m may be permitted for flights operating:
        1) at speeds that, in the prevailing visibility, will give adequate opportunity to observe 
           other traffic or any obstacles in time to avoid collision; or
        2) in circumstances in which the probability of encounters with other traffic would normally 
           be low, e.g. in areas of low volume traffic and for aerial work at low levels.
    b) HELICOPTERS may be permitted to operate in less than 1 500 m flight visibility, if manoeuvred 
       at a speed that will give adequate opportunity to observe other traffic or any obstacles in 
       time to avoid collision.
*** The VMC minima in Class A airspace are included for guidance to pilots and do not imply
    acceptance of VFR flights in Class A airspace


For Air Traffic Management purposes, ICAO Doc 4444 gives more detail of how both types of flights are treated by ATC.

National regulations may deviate from ICAO regulation. If they do deviate, the deviation have to be published in the Aeronautical Information Publication of the country.