Each example file has a block of comments at the top which highlight the main features (view in a text editor to read these). There are also two xls files in the ExampleFiles folder which summarize the example files:
ExampleFiles.xls has one row per file summarizing key characteristics and at the far right, some sentences explaining the key feature of this example.
ExampleFiles-ObjectsLink.xls lists every EnergyPlus object type and the first five example files that contain that object.
For example, if you are interested in cross flow ventilation, look for RoomAirSettings:CrossVentilation, and there is one example file which uses that object type: CrossVent_1Zone_AirflowNetwork.idf