What is the difference between Retin-A and Glycolic Acid?

Thank you for your question.  Retin A (also known by its generic name, tretinoin) is a potent derivative of vitamin A.  Although it was initially discovered as an acne medication, tretinoin is highly effective as a collagen stimulator.  There really is no other topical agent out there with more scientific studies supporting its efficacy in terms of reducing fine lines and wrinkles - tretinoin truly is the gold standard of anti-aging.  Tretinoin can be a little irritating when you first start using it - its not uncommon to experience some redness, peeling, and sun sensitivity.  Tretinoin is only available by prescription - there are milder, over the counter vitamin A creams (known as retinols) that are available without a doctor's prescription.  
Glycolic acid is a chemical exfoliant - it essentially works by gently removing the top layers of dead cells from the surface of the skin.  Glycolic acid does have mild anti-aging properties as well, and regular use does help your other skincare topicals better penetrate your skin.  
I would recommend discussing your skincare concerns with a board-certified dermatologist to determine the best regimen for you.  Best of luck!