The word 'turn' in the golf swing has been a term in golf that has been largely misunderstood over the years. Over the decades the degree of turn in the swing has changed considerably. In the early Greg Norman and Ballesteros days we saw a lot of turn and movement particularly with the lower half. As years have gone by the realization has been that in order to create a more efficient swing, less movement and turn needs to occur in the golf swing.
There are three major areas that require turn in the golf swing, these being: shoulders, hips and knees. Since the upper half starts the backswing, the shoulders move first and thus will move the farthest at 90 degrees (under the chin), next come the hips at 45 degrees and lastly the knees will move around 20 degrees.
Turn also has an impact on swing path. If you turn too much on your backswing, the club will tend to travel on an inside path and thus may cause an 'over the top' swing or a swing that starts on an extreme inside path on the way down.
Which one will be determined by your transition specifically whether you use your legs to start the downswing or your upper half. Insufficient turn often causes an upright swing and thus will become more likely for the club to travel on a vertical plane on the way down. This will result in 'fat' shots and 'slice' shots.