Calcutta is the former English name of Kolkata, a large city and port in eastern India, on the Hooghly River, and the capital of West Bengal. English speakers used Calcutta for a long time; the city later asked the world to use Kolkata, which is closer to the Bengali name. You will still see Calcutta in older books, old maps, company names, and history. Though actually, it is not Calicut — that is a different city on the other side of India, and the two names are not related. Using Kolkata is the usual polite choice now; Calcutta tells you the speaker is talking about the past, or quoting an old name.