Perhaps you've been in a tasting room and heard someone reference a wine as being done in an "old world style," or read a review about a wine that made a new world/old world comparison. What does it all mean? Which countries are new world? What IS the difference between the wine itself? Let's see if we can shed a little light on these questions.
First, let's lay out the geography for the "New World" wine producing countries. Pretty much every wine producing country in the world that does NOT fall within the continent of Europe (and parts of the Middle East) qualifies as New World. That would Australia, New Zealand, South Africa, and the Americas - North and South, mainly Chile, Argentina, Canada and the United States. Peru and Mexico, while historically relevant to wine in the new world, are no longer major wine producing countries.
The history of how wine grapes came to each of the new world wine countries varies somewhat, but is largely the result of immigration in it's various forms - whether by conquering native lands, hostile takeovers or deliberate movements of people and culture. Many of the varieties of grapes being currently grown can be traced back to European (Old World) vineyards.
Stylistically, new world wines have a tendency to be fruitier and higher in alcohol than their old world counterparts. Much of this is due to the warmer climate and lengthened growing season of many new world wine regions. New world wine regions also have less stringent rules and traditions as to varieties grown, farming practices and harvest yields, allowing for greater experimentation, in the vineyard and in the winery.
Labeling laws differ, also. New world wines are typically labeled varietally (cabernet sauvignon, viognier, or tempranillo,) as opposed to regionally (Bordeaux, Rhone Valley, or Rioja), although new world wines did borrow heavily from old world styles - Champagne and Port being the two most glaring examples. Each new world wine country has laws in place to verify the accuracy of varietal listed and the region it is from, but a malbec wine can be grown and made anywhere from Argentina to California to Texas.
The difference in new world and old world wines is gradually blurring, as old world winemakers look to maintain their traditions while exploring new styles and techniques of winemaking, and new world winemakers continue to learn the subtle differences and importance of terroir. Wine, while centuries old in it's history and cultivation and culture, is, and always will be, a living, evolving creature, no matter which part of the world you live in.