There are a number of different planes in the golf swing, and there are many different interpertations as to which are important, and where you want to be a particular points.
To help provide insight I have included screen captures of four pros (Ernie Els, Steve Elkington, Greg Norman, and Nick Price) through out their swings, and noted their positions relative to some of the planes.
Setup:
At setup I have drawn a (yellow) line through the golfers spine (so that we can see the golfer's shoulder plane relative to the spine during the swing). A white line has been drawn along the club shaft at address to show the original shaft angle plane, and a white line has been drawn to show the Ben Hogan/glass plane that is popular with some golfers.
Hip high back:
When the hands and club reach hip high in the backswing we have the next plane check point. Many instructors like to see the club head and shaft swing along the original shaft angle on its way to the hip high position. Ernie Els and Steve Elkington arrive with the hands and club head just above the line, and with the club head and hands on the same level. Greg Norman has his hands slightly higher than Els and Elkington, and the club head on the original shaft angle plane at hip high. Also worth mentioning is that the angle of Norman's clubface is parallel to his spine angle showing that he has not rotated his arms or fanned the club open as less skilled players tend to do at this position. Finally we have Nick Price who has his hands in a position like we see with Norman, but Price's clubface is just inside the Hogan/glass plane. Most instructors prefer to see students in the Els/Elkington position.
Left arm parallel back:
The next check point is when the left arm reaches parallel to the ground on the backswing. I have added a red line through the shaft and extended it to the ground to show the shaft plane angle, and a blue line to show the angle of the shoulders. With all four golfers we see that the shoulders are rotating perpendicular to their spines. With Els and Price we see the shaft and butt end of the grip pointing part way between the feet and the ball. With Elkington we see the shaft and butt of the grip pointing directly at the ball. Finally with Norman we see the shaft parallel to the original shaft angle plane and see that it points outside the golf ball. Most instructors prefer to see students in the Els/Price position.
Top of swing:
The next position we are looking at is the top of the swing. Here we have the blue line representing the shoulder angle, and the red line representing the left arm plane angle. All of the golfers have continued to rotate their shoulders pretty perpandicular to their spine angles. The shoulder planes are all on flatter (more horizontal) angles than the left arm angles (with Norman's being the closest match between the two). Els, Elkington, and Price's left arm planes are parallel to the original shaft angle planes. Also of note are the positions of the right forearms at the top. All of the pros have their right forearms parallel (or close to parallel) to their spine angles.
Left arm parallel down:
When the left arm drops to parallel to the ground on the downswing we see the shaft plane angle for the downswing. All of the pros have their shafts dropping down by the right shoulder. With Elkington and Price we see the shaft plane pointing outside the ball. With Els we see the shaft pointing directly at the ball. Finally with Norman we see the shaft pointing inside the ball. An interesting occurance you may have noticed is Price and Norman are now the opposite of what they were on the backswing (Norman pointed outside the ball, and Price pointed inside the ball).
Hip high down:
When the hands reach hip high on the downswing we have another check of the shaft planes. Elkington and Price now have the shaft pointing outside the ball and the shaft planes are parallel to the original shaft angle planes. Els and Norman have the shafts pointed at the golf ball, and the shaft is halfway between the original shaft angle plane and the Hogan plane.
Impact:
At impact Els, Elkington, and Norman all have their hands elevated above where they were at address, and the shaft is on a steeper angle at impact then it was at address. Price has returned his shaft to the original shaft angle plane, and his hands are at the same height they were at address.
Shoulder high through:
The final plane lines we'll look at are the ones we see when the golfers hands reach shoulder high on the through swing. All of the pros have their shoulders perpendicular to their spine angles (showing that the shoulders are still turning level around the spine). Els and Price have the shaft pointing slightly inside where the ball to target line, Elkington has the shaft pointing at the ball to target line, and Norman has the shaft pointing outside the ball to target line. (All of these players have the shaft pointing in the same general area, from a ball to target line perspective, that they did at the left arm parallel back position).
Hopefully this walkthrough of four pro swings has given you a better understanding of golf swing planes, and help you to better understand your own swing.