"Ground reference maneuver" is a term used in aviation. As the term indicates, it refers to any flight maneuver which is performed with reference to a point or points on the ground. The primary purposes of learning and practicting ground reference maneuvers are to teach aviators to observe and compensate for wind and to ensure that they can consistently and predictably perform maneuvers which they may be required to carry out for the purposes of traffic control. In addition, practicing these maneuvers will give them experience and confidence in maneuvering close to the ground (pattern altitude) and prepare them for the three maneuvers which are included in the U.S. Private Pilot (Airplane) practical test standards (translation: there will be a quiz and you'll be expected to do this with an FAA examiner in your airplane - remember your driving test? This is worse).
The rectangular course is one of the earliest ground reference maneuvers a student pilot will learn, because they'll use it nearly every time they fly. The traffic pattern used to help ensure that multiple aircraft can safely use an uncontrolled (or controlled) airport is a rectangular course. The course is divided into four legs, and they are named the crosswind, downwind, base and upwind. In the traffic pattern, the 'upwind' generally comprises final approach and climb out, as the runway is aligned along the upwind leg. To fly a rectangular course properly, the pilot will need to adjust the bank angle used at each corner and the heading flown on each leg. Let's look at it from the point of view of an airplane taking off, flying the pattern and returning to land - this is called closed traffic or remaining in the pattern. Let's assume that the airplane is taking off directly into the wind. On climbout, the airplane will fly a heading directly into the wind, not adjusting to left or right. When turning crosswind, the pilot should not turn a full 90 degrees but should instead roll out of the turn early in order to maintain a wind correction angle with the nose of the airplane slightly into the wind. This will result in a ground track that is directly perpendicular to the runway, as the angle will compensate for the wind from the right. In addition, since the ground speed is slowest during this point, the bank angle should be slightly shallower than standard in order to maintain the proper ground track.
To turn downwind, the pilot will need to turn slightly more than 90 degrees (to include their wind correction angle). However, ground speed should be such that a standard rate turn can be used. In the real world, traffic patterns are large enough that standard rate turns, held slightly longer, will suffice. On the downwind, the airplane uses no wind correction angle.
Turning base, the pilot will need to turn more than 90 degrees again in order to reach a heading with a sufficient wind correction angle to maintain a perpendicular base ground track. In this case, since ground speed is faster, the turn will need to be at a slightly more than standard bank angle in order to maintain a consistent ground track.
Turning final (onto the upwind leg) the pilot will need to turn less than 90 degrees once more, and the bank angle should be medium. After enough practice, this will become almost automatic; the pilot will be able to observe his or her ground track (these maneuvers are generally flown at or below traffic pattern altitude, 1000 feet above ground level) and make wind corrections from experience.
Turns about a point
At times it will be necessary (or expedient) to maintain a circular course about a fixed point. Air traffic control, if you fly IFR, may ask you to 'orbit' a fixed point. Even VFR flights may be asked to orbit particular points if air traffic control determines that they need to delay the progress of the airplane for a time. The goal is to ensure that you remain where ATC thinks you are, and not drift or precess downwind! This maneuver is slightly different from the rectangular course. Since no straight legs are flown, wind correction angles will not apply. Instead, the pilot will adjust the bank angle as he or she orbits to compensate for the wind direction. If the airplane is flown in a circle about the reference point, when the aircraft is pointed directly upwind, and is to the right of the reference point (these maneuvers are generally done counterclockwise, as the pilot in command sits on the left side of the airplane and thus it is easier for him or her to see a point on the left side of the airplane) then the bank angle will be a standard rate turn. As the airplane progresses towards upwind, the bank angle becomes shallower, ensuring that the airplane retains more upwind travel; once the airplane has reached the upwind side, begin increasing the bank angle for the second half of the circle.
S-Turns (sometimes called 'S-Turns along a road')
As the name implies, this maneuver involves crossing a straight reference line on the ground (a road, railroad or power line cut are the easiest to find) with the wings perpendicular to the line. As soon as the airplane crosses, the pilot begins a turn to the left or right. The goal is to cross the line going in the opposite direction with your wings perpendicular once more, and then turn the opposite way to continue 'down' the reference line in half-circle courses. In order to do this, the pilot will (as in the turns about a point) need to constantly adjust the bank angle to compensate for wind.