What is the difference?

All the jabbering and quality classification aside, it's as simple as this:
Psych rock is how it started. When it became popular and the parents wanted to slap a label on it, they called it Acid rock. That simple. There is no difference in all honesty, it's just a label.

I always though they were the same thing.

THEY ARE!!!!!!!!!

For me, Frampton Comes Alive is acid rock. For me and my homeboy (literally) Ivo anyway.

What the hell is Acid Rock?

It's when a bunch of people on acid come together and try to make rock music.

That's the point I was trying to make originally. Acid Rock is mainstream Psych Rock. To answer a previous question, I basically consider Psych Rock to be any band that started the movement underground like Floyd and The Mothers of Invention. Acid Rock came about when bands like Jefferson Airplane and even Cream jumped on the bandwagon.