Before you talk about specific wines (like Pinot Noir or Merlot, as opposed to specific producers), you really have to start with a discussion of grapes. While there are lots of grapes in the world (travel up and down the "Central Valley" of California and see all the "table" grapes), there are a select few which are used for making the best wines. These are known as "noble grapes.".
A note--I know that there are a lot of grapes missing. As time permits, they'll get added. Here are some:
Cabernet Sauvignon. One of the components of French Bordeaux, it is also the major (if not sometimes only) grape in the most popularly drunk American red wines in what might be called, for lack of a better term, the "snob appeal" class. (For in fact there is probably more American jug wine that never sees the cabernet grape drunk each year in the United States than all the cabernet sauvignon from all the wineries in the world put together. Prestige and/or quality are not always equal to popularity.) Cabernet Sauvignon contains a lot of tannins that lead to the long aged, "better" red wines. Depending on where it is grown it may smell of cassis and black currants or black cherry and red currants. Some people may notice a cigar box smell. Bell peppers, asparagus, and rhubarb are common tasting notes for cabernet produced from grapes that are not quite ripe. A bit of this sort of character is considered, by some, to be pleasing (the wine is called "herbaceous"), too much of this flavor is unappealing--and the wine will be described as "vegetal". Out tasting at a "fancy" winery I tasted a wine that smelled and tasted so overwhelmingly of asparagus (which I don't like) that I couldn't drink anything else the entire day. The winery people admitted that while some people loved that particular wine, others had the same reaction as myself. I think I turned about as green as the asparagus I imagined.
Barbera. A major Italian variety with a "tarry" smell and medium body.
Cabernet Franc. Also a component of Bordeaux, a little is often blended with Cabernet Sauvignon to add bouquet. Some don't think much of it when drunk all by itself.
Gamay. Produces a fruity wine such as French Beaujolais. (The California Gamay Beaujolais is not the same grape, but makes a wine that comes close.)
Grenache. Often used to make rose wine, it is a component of French Chateauneuf-du-Pape, Cotes du Rhone and most other appellations from the south of France. There are also many tasty grenache-based wines from Spain (where it is called garnacha) and from California.
Merlot. One of the major components of most French Bordeaux, also with less tannin that makes for a smoother characteristic in the wine. Alone (or practically alone), it makes another of the more popular U.S. wines. Though it is like Cabernet, it is usually "rounder". It is often blended with Cabernet Sauvignon.
Nebbiolo. Can be found in California, but is really a grape of the Piedmont area of Northern Italy. Found in Barbaresco and Barolo wines, which can be aged with great success.
Syrah. "True Syrah" and Petite Sirah are not the same, the former a relative of Durif from the Rhone in France (and a major variety in its own right), the latter a variety grown relatively widely in California and said to be genetically the same as the obscure French Durif variety. Both produce more or less deeply-red-colored, tannic, long lived wines, the latter being a bit more "peppery." You might also see Australian Shiraz, which is the same grape variety as the "true" French syrah, but because of differences in growing conditions between the two countries, much of it ends up tasting more like the California petite sirah, perhaps with more of a chocolate note.
Pinot Noir. The only grape in the famous French Red Burgundy appellations of the Cotes de Beaune, Cotes de Nuit and Cote d'or.. Some U.S. winemakers will make Pinot Noir "in the French style." Or not. Interestingly, they are lighter in color (but not flavor) than Bordeaux/Cabernet.
Zinfandel. Mostly from California, it has a great deal of fruit like characteristics. Some young Zinfandels are also "spicy." Good red Zinfandel is often a bargain in restaurants, being less expensive than other wines, but still very drinkable. (Huge quantities of Zinfandel are made into "White Zin," a sweet, uncomplicated (and usually inexpensive) wine that is favored by people who do not drink much wine. A decent White Zinfandel can make a nice "picnic wine." We especially like zin from "old vines" (pictured).
Chardonnay. Produces French white Burgundy and perhaps the most popular (once again "snob" class--see Cabernet Sauvignon, above) wines in the U.S. "Give me a glass of white wine" will probably get you Chardonnay at "better" restaurants. (In fact, a lot of jug wine--which is to say, a vast amount of wine--in the United States is made from what are "lesser" grape varieties like French colombard or sultana.)
Chenin Blanc. The major grape planted in the French Loire valley. In the U.S., often used to make a light, fruity wine.
Gewurztraminer. Some confusion abounds this wine, partly because non-German speaking persons may not order it in a restaurant because they can't pronounce it and partly because of the way in which parts of the word can be translated. I'm told the German word "wuerz" literally means "spice", but "gewuerz" is better translated as "aromatic" or "fragrant." Wine from this grape has a floral smell and the wine itself is often drunk with spicy foods. Gewurztraminer also makes a good "late harvest" sweet dessert wine. It is more common in Alsace, Italy, and the United States than in Germany and many "experts" say Alsace makes the best.
Riesling. Also, to me, producing a floral smelling sort of wine, it also makes a sort of light, fresh type of wine. Makes a great "late harvest" sweet dessert wine (for which it is especially known in Germany). Another viewpoint, it isn't so much floral as "minerally" with accents of fuel oil--not light and fresh, instead, lots of depth and complexity in something like a good German Riesling Spatlese or Alsatian Grand Cru.
Sauvignon Blanc (sometimes called Fume Blanc, at least in California). In the U.S., makes a crisp, light wine (sometimes with a "grassy" or "herbaceous" characteristic). It is a component (along with Semillon ) of the French dessert wine, Sauternes and the white wines of Bordeaux.
Semillon. As with many grapes, while grown elsewhere (such as California), Semillon is one of the major varieties grown in Bordeaux. Like Sauvignon Blanc is can often have a grassy (or herbaceous) note, but also may have notes of ripe figs. It may be drunk "dry", or "sweet", and as such, it is a component (along with Sauvignon Blanc ) of the French dessert wine, Sauternes and the white wines of Bordeaux.
Because I am neither an expert or a global traveler, nor independently wealthy, you may notice a lack of discussion about other grapes from around the world. I'm always open for opinions, though! Anybody want to tell me a lot of good things about, for example, Germany, Italy, Australia, New Zealand, South Africa, Spain (these are things people have written to me about) and you name the list of other countries, wines, etc. that I've missed!