I have gone through many documents, but don't understand what a perfect fifth is. Can somebody please explain with an example? (An example is important!)

I have already found these explanations, but I don't understand what they mean:

> 
> In music theory, a perfect fifth is the musical interval corresponding
>  to a pair of pitches with a frequency ratio of 3:2, or very nearly so.
> 
> 
> 

...and this one...

> 
> In classical music from Western culture, a fifth is the interval from
>  the first to the last of five consecutive notes in a diatonic scale.
>  The perfect fifth (often abbreviated P5) spans seven semitones, while
>  the diminished fifth spans six and the augmented fifth spans eight
>  semitones. For example, the interval from C to G is a perfect fifth,
>  as the note G lies seven semitones above C
> 
> 
>