Many people refer to the Netherlands as Holland, but do these two terms really mean the same thing? The short answer is no, although even many Netherlanders (including the Netherlands Board of Tourism and Conventions) refer to their own country as Holland.
Once upon a time, Holland was an independent country with its own government, language, and currency. Beginning in the 16th century with the Eighty Years' War, the area around Holland — which included, among others, the Northern and Southern Netherlands, Friesland, Zeeland, and Utrecht — underwent many political transformations. Wars, monarchical changes, treaties, and politics turned this area into the Dutch Republic, the Batavian Republic, the Kingdom of Holland, the principality of the Netherlands, and finally into the Kingdom of the Netherlands.
Throughout all this turmoil, the major cities of Holland — among them Amsterdam, the Hague, Rotterdam, and Haarlem — remained major European cities of commerce and banking. As traders and explorers from Holland journeyed across the globe, the people they came in contact with learned the name Holland first, and the name, to a large extent, stuck.
The Kingdom of the Netherlands (Koninkrijk der Nederlanden) is made up of three constituent countries: The Netherlands in Europe; and, in the Caribbean, Aruba and the Netherlands Antilles (the islands of Curaçao, Bonaire, Sint Eustatius, Saba, and Sint Maarten).
Holland refers specifically to two provinces in the central-west Netherlands, North Holland and South Holland, which still encompass the largest cities in the Netherlands. Although Holland is considered by many the "official" unofficial name for the country of the Netherlands, some Netherlanders take offense at its usage. Although people will know what you're talking about if you call the Netherlands Holland, it's a bit like referring to the former USSR simply as Russia.