Scale Length : The measurement from the face of the nut on the fretboard side to the point on the bridge where the string passes over the saddle.

This definition is good, if you are casually refering to scale length, but does not provide an accurate scale length measurement.

To accurately measure scale length, take the measurement from the face of the nut to the 12th fret wire and multiply by 2.

Definition

Why is compensation necessary? When fretting a note you push the string down a fraction of an inch until it meets the frets. This distance increases the tension on the string and causes the note to go sharp.

More info on compensation can be found on the web. Recommended sites

William Cumpiano Guitars: Authur of Guitar Making Tradition and Technology his website has updates on the book and has other information about guitar making.

Buildyourguitar.com: Calculating compensation and other helpful guitar building tips

Determining what scale length is right for you.

A longer scale length will have a higher string tension, and because the frets are larger will be easier to play if you have larger hands.

A shorter scale length will have lower strings tension and will be easier to play if you have smaller hands.

* String guage will also have an effect on string tension. On a longer scale guitar, using a light string guage might make notes hard to fret and bend. On the other end of the spectrum, a shorter scale guitar with heavy guage strings will make the strings too loose and will cause the strings to buzz.

Authors experience, I have always played a guitar with a longer scale length and aquired a guitar with a shorter scale length. After playing the shorter scale length guiar I noticed that I had a lot less cramping and fatigue in my fretting hand and arm. The other difference was the ease of bending strings and fretting notes. Everyone is different and has thier own prefrences, try different instruments and make a decision based on your own experience.