Washington Post national security reporter Dan Lamonthe passed along this Department of Defense map showing where exactly in Syria the US dropped bombs and launched cruise missiles on Monday night:
This map just shown at the Pentagon of last night's military strikes in Syria. pic.twitter.com/q6bJVUqIIL— Dan Lamothe (@DanLamothe) September 23, 2014
If you compare the map to this one by Vox's Joe Posner, showing areas held by ISIS, the overlap is readily apparent. See particularly the strikes around the northern Syria city of Raqqa.
It appears to include Raqqa, the self-declared headquarters of ISIS, as well as other strongholds like Dair Elur, Hasakah, and Abu Kamal. Some strikes also targeted al-Qaeda militants in Syria with Jabhat al-Nusra and a group the Pentagon says is called Khorasan. For more background, see Max Fisher, Zack Beauchamp, and Amanda Taub's guide to what we know and don't know about the strikes so far.