Peninsular is usually a describing word. It means "of a peninsula" or "like a peninsula": land with water around almost every side, still joined to a bigger land. So a peninsular town is a town on that kind of land; peninsular weather is the weather there. It is more formal than peninsula. Though actually, the land itself is a peninsula. Peninsular hangs on another word and describes it. In history you may see Peninsular with a capital P, as in the Peninsular War in Spain and Portugal. In older writing about the Spanish empire, a peninsular could also mean a person born in Spain, not in the Americas. That use is special and not the one most students need. For learners, this word points at peninsula-land or at things from such a place. It is not the usual name of the land.