Tannins are among the most important polyphenolic compounds in red wine and play a central role in determining color stability, flavor complexity, and aging potential. This review systematically summarizes the classification, structural characteristics, and the distribution and transformation mechanisms of tannins in red wine. It focuses on the differential effects of soft tannins, hard tannins, and polymerized tannins on the formation and maintenance of wine color, as well as their roles in astringency, bitterness, aroma coordination, and flavor persistence. Furthermore, the applications of tannins in fermentation, aging, and blending are summarized, revealing their important roles in promoting anthocyanin stability, enhancing wine body structure, delaying oxidation, and improving mouthfeel.