Modern pianos are usually tuned for equal tempering, which simply means that all of the semi-tones, or half steps within an octave actually have exactly the same relative difference in frequency. Each half step is 1.05946309.... (the 12th root of 2) higher than the previous note. Modern guitars are typically fretted and tuned to the same principles.

Much of the baroque or earlier music was originally written for earlier tempering schemes, and just didn't use notes outside of the specified scale. When singing or playing ensemble pieces from those eras, or even when with modern music, the troublesome 3rd of a chord is often bent to get better harmony.

This site below has some fairly concise explanation and examples:

The Mathematics of Tuning and Temperament

(in case the blue clicky thing doesn't work)

http://www.hlalapansi.demon.co.uk/Acoustics/MusicMaths/MusicMaths.html

Nowadays we are so used to the equal tempered piano that we tend to ignore the nuances of the old perfect tunings.

~S~