As you know, when you fly a 737-400 and the ATC tells you the instructions you can almost always choose another approach, but they sometimes tell me to circle to land.
How am I suppose to circle then?
They will say something like this, "American 782, you are 65 miles south, turn right heading 245 expect vectors ILS 27R. Circle to land runway 9L. That means to fly the ILS approach for 27R and instead of landing on 27R circle the airport until you are aligned with 9L. I don't think they do that in real life. Hope that helps.
Does happen but not that often. Used generally in IMC when the wind favours a runway that has no instrument approach. One descends on the ILS for example then once visual and able to stay visual (below cloudbase) you circle onto the rw in use.
PH's explanation was correct but incomplete.
There are also many circle-to-land- approaches that are executed every time the approach is made. These occur where there is no nav aid in a position to allow a straight-in approach. Many, many VOR approaches are in this category. In this approach, you would fly the published course from the nav aid until the airport was in sight. You would then circle to align yourself with the landing runway. It is the pilot's obligation to always keep the airport in sight from the point where he/she leaves the published course and begins to circle to the runway. If the pilot ever loses sight of the airport, they must execute an immediate missed approach. Circle-to-land approaches always have higher MDA's (minimum descent altitude)than do straight-in approaches.
An example is the VOR approoach to Logan-Cache, Utah airport. If you get that chart, you'll see that the approach heading from the VOR is 156 degrees. Once the airport is in sight, you must then circle to land on whichever runway is in current use: RW17, RW35, RW28, or RW10. The MDA is published as a "circling" minimum.
Although this is a circle-to-land approach, it will be issued by ATC as "cleared for the Logan-Cache VOR approach" since circling is part of the published approach. ATC will clear you to a circle-to-land approach when the published approach is to a specific runway other than the one of intended landing.
That is somewhat what I meant.