Aspirin is a chemical called acetylsalicylic acid that was first discovered and sold by the German company Bayer in the late 19th century. It is composed of a benzene ring as its central structure. One of the carbons of the benzene ring has a carboxylic acid (COOH) attached to it. One of the adjacent carbon atoms on the ring has an acetoxy group (an acetyl ester CH3CO2). And that is it. Aspirin has a surprisingly simple structure. It's main metabolite is salicylic acid, which is the same structure but instead the acetoxy group has been hydrolyzed to simply the hydroxyl group (OH). The chemical structure can be seen drawn in the first link below.