Okie-dokie, before we get started, it’s probably best to get the pronunciation of terroir word out of the way before we move on with the rest of the article: Terroir [ter-whah]
Very loosely translated from French (since no-one can agree on the exact definition, and there is no English equivalent), terroir means "a sense of place". It's applied by wine nerds to talk about all the things that have an effect on the grapes in a vineyard. So the weather, the soil, hours of sunlight, the slope of the hill where vines are planted, the macro climate of the region as a whole, right down to the micro-climate in the individual rows of grapes.
To understand the concept of terroir is to believe that all of these factors combined are going to have a effect on the taste of the wine, and that there's no possible way that the wines from a particular region can be duplicated.
Of course, this is based on the fact that every vineyard in the world has its own individual “terroir”. So the question is: Does each parcel of land, within each vineyard, within each region, within each country, really produce wines that taste completely unique from other surrounding vineyards? The answer is: Potentially, yes; realistically, no.
The concept of terroir depends a whole bunch on the dude making the wine, and the taste profile they’re looking to achieve.
European winemakers take the aspect of terroir very seriously; for example, wine from a Grand Cru vineyard in Burgundy will cost a ridiculous amount more than a Premier Cru vineyard, even though the two plots of land can be literally feet away from each other, and made by the same winemaker. But, maybe the Grand Cru site has better drainage, more pronounced minerality in the soil, or gains that extra little touch of sunlight every day because of the slope of the hill etc.
The thing with terroir is that it goes completely out the window if the winemaker applies a "heavy hand", as New World producers frequently get accused of, i.e. oaking the arse out of a wine, grapes being heavily ripened before harvest; and if certain additives/fermentation techniques are applied, a Chardonnay from Australia, will taste like one from Chile, will taste like one from California, will taste like one from France...