Room Treatment

+1 winoguy -- "acoustic panel" is not specific about whether it diffuses or absorbs.

IME, Acoustic panels are primarily designed to absorb sound waves to reduce room echoes and reverberation, while acoustic diffusers scatter sound waves to create a more balanced and natural-sounding acoustic environment. Check out some of the very best rooms in Audiogon Virtual systems to have better understanding and placement of these two very critical but different acoustic treatments. 

@lalitk It's hard to get clear on this for a lot of folks. If you go to Acoustimac's website, it aligns "acoustic panel" with absorption. If you go to GIK, the first things displayed are diffusers but they also have absorption on the page.

Viz. https://www.gikacoustics.com/product-category/acoustic-panels/

I see GIK and other sites like it have panels for absorption, and Decware has diffusers which look like window slats or turning vanes, and they recommend one for each wall. I was looking to see if anyone has experience with both, and what application works best for a standard 15x15 listening room with gypsum wallboard walls.

A "panel" is a generic term. 

Panels can be absorbers, diffusors or a combination.  There are also bass traps which are a type of absorber that is especially effective in the bass region. 

GIK makes some panels which are a combination with absorption materials behind a diffusion grid.  This allows for high frequency diffusion along wiht mid range absorption.