What is the difference between dark spots, blemishes, acnes, scars & pimples? How do they form? Which skin type suffers what? Over the counter drugs?

Manage it. Multiple pimples is usually acne; blemishes is a "pop" euphemism for acne and it leaves dark spots. Before you or someone you care about ends up with scars, manage it. Forget about "skin types" or trying to figure out "why". If topical benzoyl peroxide or retinoic acid hasn't cleared you in two weeks, you need to add prescription-strength Rx from your physician.