WHEN XL=XC AT THAT TIME THE RESONANCE WILL OCCURES

At resonance only resistence is prasent.
Capacitive impedence is equal to inductive impedence

in resonance XL = Xc
      two type:
                   series - current is min.Z in max.
                   parallel- vice versa

in resonance XL = Xc
      two type:
                   series - current is min.Z in max.
                   parallel- vice versa
and the P.f = 1 bcz R=Z

resonance is the tendency of a system to oscillate at 
maximum amplitude at a certain frequency. This frequency is 
known as the system's resonance frequency (or resonant 
frequency).

it is used for tuning the signals in electronics and 
producing control for damping in electrical circuits

In electrical circuits, resonance occurs when the inductive 
and capacitive reactances are equal and cancel. If the 
reactors are in series, the circuits' impedance becomes 
minimum and equals only the circuits' resistance. The 
circuit current achieves a maximum value. 

If the reactors are configured in parallel, often called 
a "Tank" circuit, the resulting impedance becomes maximum, 
ideally infinite, and the source current passing through 
the parallel reactors becomes minimized, ideally zero. 
Within the tank circuit however, the current achieves a 
maximum value due to the cancelling reactor values.

when a body is vibrated with some periodic force,if the 
natural frequency of that body coincides with the frequency 
of the periodic force applied on it, with maximum amplitude 
it is known as resonance......there are many types of 
resonance-optical, electrical,mechanical,etc............