There are three official tiers of Italian wines. They are DOC, IGT, and DOCG

DOCG (Denominazione di Origine Controllata e Garantita) is the highest classification for Italian wines. It denotes controlled (controllata) production methods and guaranteed (garantita) wine quality. There are strict rules governing the production of DOCG wines, most obviously the permitted grape varieties, yield limits, grape ripeness, winemaking procedures and barrel/bottle maturation. Every DOCG wine is subject to official tasting procedures. To prevent counterfeiting, the bottles have a numbered government seal across the neck.
Sardinia

DOCG: Vermentino di Gallura
DOC (Denominazione di Origine Controllata) is the main tier of Italian wine classification, and covers almost every traditional Italian wine style. There are around 330 individual DOC titles, each with a set of laws governing its viticultural zone, permitted grape varieties and wine style. Those which show consistently high quality earn promotion to DOCG status.
Sardinia

DOC: Alghero, Arborea, Campidano di Terralba/Terralba, Cannonau di Sardegna, Carignano del Sulcis, Giro di Cagliari, Malvasia di Bosa, Malvasia di Cagliari, Mandrolisai, Monica di Cagliari, Monica di Sardegna, Moscato di Cagliari, Moscato di Sardegna, Moscato di Sorso Sennori, Nasco di Cagliari, Nuragus di Cagliari, Sardegna Semidano, Vermentino di Sardegna, Vernaccia di Oristano

IGT (Indicazione Geografica Tipica) was introduced in 1992, to allow a certain level of freedom to Italy’s winemakers. Prior to 1992, many wines failed to qualify for DOC or DOCG status – not because they were of low quality, but because they were made from grape varieties (or blends) not sanctioned under DOC/G laws. The IGT classification focuses on the region of origin, rather than grape varieties or wine styles.
Sardinia

IGT: Barbagia, Colli del Limbara, Isola dei Nuraghi, Marmilla, Nurra/Nurra Algherese, Ogliastra, Parteolla, Planargia, Provincia di Nuoro/Nuoro, Romangia, Sibiola, Tharros, Trexenta, Valle del Tirso, Valli di Porto Pino