WSTR/Atlanta "Star 94" was a good example of an Adult Top 40 up until about 5 months ago. They basically took the Top 40 chart and picked out the songs that had an overall musical flavor to them. In other words, they left out anything that remotely sounded like rap. If there was a hip-hop song that was a monster hit on Top 40 (such as Usher's "Yeah"), they would edit the song heavily to remove any parts where the words were rapped instead of sung. Also their recurrents stayed in the rotation a lot longer than a typical Top 40 station. Add in some late 90's Hot AC gold during the day and you've got yourself an Adult Top 40 according to Star 94/Atlanta.
Of course now they would not be a good example of one. In recent months they have gotten a lot harder as competition has heated up in Atlanta. Their main competitor is WWWQ/Atlanta "Q100", whose daytime playlist is a great example of an Adult Top 40.