Common scale lenghts are 24 3/4" and 25" but what does that mean? It's really quite simple. The scale length of a guitar is the distance measured along the centerline, from the top of the fretboard (the headstock end, where the nut meets the fretboard) to the point where the strings touch the bridge saddles.
One might say "well that doesn't sound so simple, the bridge saddles on my guitar are all in different positions," which would be a good observation. Adjustable saddles allow us to intonate our gutar so that it is as close to being in tune as possible. Different string guages result in different saddle positions for optimum intonation, so the saddles might get adjusted and readjusted if different guage strings are used on the same instrument.
Every neck is also made for a specific scale. If you happen to find a stray neck laying around, and decide you're going to build a guitar out of it, you'll need to figure out the scale lenghth of the neck to properly place the bridge.
Well, don't sweat it, that's easy too! Just take out your trusty ruler or tape and measure the fretboard along the centerline from the nut to the middle of the 12th fret (the middle of the 12th fret itself, not the middle between the frets). Muliply this number times 2 and this distance will be your scale length. Once you've determined your scale you can insert or attach your neck and mark the routes for the bridge.