i have read some pages .there i seen that some say that capacitor allow dc and block ac and some say blocks dc and allow ac.which one is right which is really series problem to me.pull me out of this.thank you in advance

 

Depends on how the capacitor is connected.
A capacitor in series blocks DC.

A capacitor in parallel (between signal and ground) removes AC and allows DC to go to next stage.

 

Sometime ago I find this humorous drawing about capacitor.

 

Humorous but does not tell the true story.
Electron charge does not 'slip through the plates'
An Electron charge of opposite polarity gathers on each plate.
With an AC current the charge reverses during every cycle, so there is a constant inflow of current.
Max.