If you order a sundae with hot fudge and get thin chocolate syrup, you'll certainly know the difference. Syrups are typically runnier than hot fudge and may, but not always, have a weaker chocolate flavor. Chocolate syrup pours easily from a bottle and mixes readily into cool items like milk. Fudge sauce is usually thick, especially at room temperature, and often offers a more intense flavor than chocolate sauce. Heating fudge sauce to create "hot fudge" makes it pourable and offers a pleasing textural and temperature contrast to cooler items, such as cakes or ice cream.
Chocolate syrup usually consists of unsweetened cocoa powder, corn syrup or cane sugar, water and other flavorings. The thicker consistency and richer flavor of hot fudge results from a list of ingredients that includes cream, sugar, bittersweet chocolate or cocoa, butter and vanilla. Both are available commercially, but making either at home is relatively easy.