Some people may be quite confused when I use the terms downwind, final, or base. These terms come from a common flight path around an airport called the traffic pattern. The FAA's AIM (Aeronautical Information Manual) defines the traffic pattern as "the traffic flow prescribed for aircraft at, taxing on, or taking off from, an airport". The traffic helps pilots in high traffic areas by helping them set up for a good landing. Imagine of there was no traffic pattern; airplanes would be approaching the runway from every direction. Now imagine the confusion this might cause. By using a traffic pattern, operations at an airport become uniform and consequently safe.
Let's start by examining the traffic pattern from the standpoint of an aircraft taking off. For this example, we will be departing runway 21 (the airplane will start by the number 21 and accelerate towards the number 3). The airplane will climb on whats called the upwind leg. Once the airplane has reached a safe altitude (I use 500 feet as a general rule of thumb), the pilot has a decision to make. He/she can either choose to leave the pattern (perhaps to visit another airport) or remain in the traffic pattern (if he/she wants to stay at the airport maybe to practice landings). If he/she wishes to leave the pattern, the pilot can either continue straight forward, or turn 45 degrees left or right. If you decide to head back to the runway, a left 90 degree turn is made onto the crosswind leg once reaching a safe altitude. While continuing on the crosswind leg, the airplane is continuing to climb to traffic pattern altitude. The traffic pattern altitude varies between each individual airport. Judging when to turn onto the downwind leg is a tad difficult at first. The general rule while in the traffic pattern is to always remain within gliding distance of the runway. The star situated along the downwind leg is called the downwind key. This is an imaginary point situated directly across from the runway numbers. At this point, the pilot performs operations to prepare the plane for landing such as lowering the landing gear and slowing the engine. Just like from crosswind to downwind, the turn from the downwind leg to the base leg is up to the pilot. The same situation goes for the turn from base to final. While flying on the base and final segments of the traffic pattern, the pilot begins to descend. Once on the final leg, the pilot aims the nose towards the runway numbers and lands the plane. Thanks to the traffic pattern, another pilot and aircraft have landed safely. When entering the traffic pattern, the airplane enters at a 45 degree angle to the downwind leg as shown by the diagram. When entering the pattern the plane will already be at the specified pattern altitude. Once situated parallel on to the runway on the downwind leg, the pilot continues the same procedures aforementioned.
This, of course, is just one example of a traffic pattern. Depending on the airport and it's surrondings, the pattern can be modifyed for specific situations. Information about the traffic pattern can be found in it's A/FD (Airport/Facility Directory). Thanks for reading!!!