Ayakashi are lesser spirits such as ghosts and apparitions, or simply an entity that gains spiritual powers because of whatever reason. Ayakashi can only be seen by people or individuals with strong spiritual powers. Some older Ayakashi can attain higher status and become patrons of different things like the rain or harvest. Youkai tend to be stronger, and preferably demons, they can even become human-like and appear in the human realm. The stronger youkai can be considered gods/goddesses or can have a hierarchies set up with bloodlines like royalty. The power ladder would be based on how strong the youkai and its associates are. Youkai are more uncommon than Ayakashi. These are some basic facts, but in all honesty it all depends on what anime you are watching. In XXXholic like in Natsume they focus more on Ayakashi rather than youkai. In Inuyasha they make everything youkai and never once use Ayakashi. Most of the time they just manipulate things to fit the story-line. Maybe you'll find a youkai serving an Ayakashi and no explanation given. It's whatever the author makes it.