Puccini = Opera
That's not right, but it's a starting point. An astonishing number of operas that are performed today are by Puccini. This is perhaps his greatest work, which premièred on this day in 1904. It's his Best Work. Perhaps.
Of course, Puccini is not the sum total of opera, and Madame Butterfly is not the sum total of Puccini. But it's not a bad start at all, which is why you hear this all the time.