What is a "Grape Variety"?

Its easy to forget that fundamental questions are often not simple questions.
In hindsight, it seems like a rather sensible question: “What does the term ‘grape variety’ mean?”. At the time, I was rather startled by how difficult it was to offer a clear answer.

With a very few exceptions, all grapes used to make wine belong to a single species: vitis vinifera. Through mutation – and later, breeding – grape vines have developed various sets of physical traits (colour, skin thickness, leaf shape, etc).
Into what would otherwise be a morphing jumble of traits, steps our friend the vigneron (fr: grape grower). Wanting to plant a vineyard, he selects a vine for its particular set of traits. By cutting and grafting (asexual reproduction), a vineyard is filled with (virtual) copies of the original vine. Now we have a vineyard full of vines which share a set of physical traits. Now we have a “variety”.
Analogies can be helpful:
All dogs belong to one species (canis familiaris). However, they have been bred – with the aim of selecting particular sets of physical traits – into recognizable “breeds”. Muscat (grape variety) and Golden Retriever (dog breed) have more in common then their approximate colour.
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So, can knowing the grape variety help us know what to expect from a bottle of wine? In wine-speak: are varietally labeled wines dependable for their typicity?
Yes... and no.
This time, let’s start with an analogy:
Apples, like grapes, come in many “varieties” (in this case, the proper term is actually cultivar). Each cultivar has a distinct set of physical traits. For example: Granny Smith apples are green, very crisp, high in acid, require mild growing climate, and its slices are relatively slow to brown.
Given some practice, its not hard to imagine being able to identify various apple cultivars in a “blind tasting”.
But what about identifying the apple cultivar baked into an apple pie? Pastry recipes, amounts of cinnamon, etc. would certainly complicate matters.
Expecting all pies made from Granny Smith apples to taste alike, is rather like expecting the same from all wines made from Merlot.

Translation:
Yes, grapes of any particular variety have characteristics which will effect a final wine.
Tannat grapes – having very thick, dark skins – produce wines which are deeply coloured, strongly flavoured and tannic.
Riesling, even as it ripens, retains a high level of acidity, and – with age – its wines develop an odd aroma of kerosene.

But...
(1) Some grape varieties are very particular, but others (Chardonnay, for example) are grown successfully almost right across the world’s vineyards. Two genetically identical vines, growing in the conditions (climate, soil type) of two very different locations, will produce two equally different vats of grape juice.
One simple (and somewhat simplified) example: cool-climate Chardonnay grapes will be “crisp”, light bodied and high in acidity... warm-climate Chardonnay will be “plump”, medium bodied, low in acidity and higher in alcohol.
--Partly for this reason, many wines are labeled according to geography rather than grape variety. Most French wines are classified this way; red wines from Burgundy – despite being 100% Pinot Noir – will be labeled with a place of origin (for example, “Cote Nuits Villages”).--

(2) Like geography, winemaking technique can make a world of difference.
Was wine fermented and/or aged in oak barrels? What kind of yeast(s) were used to ferment the juice? How high were temperatures allowed to rise during fermentation take place? How long were the skins, seeds (and sometimes stems) left to soak in the vat? And so on...

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So what good is a varietal label?

Grape variety can provide one starting point for exploration.
If you find yourself enjoying a Niagara Chardonnay, move on to a Chardonnay from Chablis(France), or California, or South Africa... Learn a little about the ones you find to your liking – what do they have in common? Cool/warm-climate? Oak/unoaked?
Please be sure to branch out from your original grape variety. Extending our example: If you find you enjoy a few cool-climate, unoaked Chardonnays, try other crisp whites (Italian Pinot Grigio, Alsatian Riesling, Loire Sauvignon Blanc, etc).

And besides, everything marketed needs something on its label; In this context, I’d prefer to have no more penguins, cats, and hippopotami than absolutely necessary.

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Want to taste some contrasts a single grape variety can produce?

(1)Pinot Gris/Pinot Grigio
In northern Italy, this grape is called “Pinot Grigio”. It is harvested quite early to produce a crisp, acidic style.
(I like the one produced by Collavini... $14)
In Alsace, the same grape is called “Pinot Gris”. The style is to harvest later, producing a richly flavoured and (sometimes) almost oily textured wine.
(To be honest, I’m still looking for a Pinot Gris with which I’m really happy... aware that this will sound like faint praise, I was not unhappy with: Domaine St.Remy. Pinot Gris Reserve $18).

(2)Some Merlots call up words like “pretty” or “feminine”. Others are massive hulks of super-ripe fruit. For an idea of what I mean, compare these two wines:
“Merlot” Christian Mouieux. Bordeaux, France ($15) (any vintage except the unusually hot 2003)
“Hillcot Merlot”. Grant Burge. Australia ($20)