Fun definition: the definition of a good wine is the wine you like to drink.

Real definition: the difference between good wines and bad wines is the complexity of the flavor. Crap wines usually have a very narrow and uncomplicated flavor profile, wines that are more likely to be enjoyed have more complex profiles. What makes it confusing is that different kinds of wines have radically different flavor profiles: a Pinot Noir has a "lighter flavor" than a Cabernet Sauvignon. The $2 a gallon wines at the local gas station are unlikely to have a complex flavor profile, and that's why people call them "crap".

Also, contrary to popular belief, wine blends aren't necessarily a sign of a bad wine, due to local soil conditions (which provide different nutrient ratios and can change the flavor profile quite a bit) some varieties of grapes grow fine, but don't produce the desired complex flavor profile. Vintners blends wines from several sources like a recipe to "build up" the flavor profile they wish for.

To learn to drink wine, simply try lots of them, try them by themselves and with different foods. Some are better paired with certain foods (the food diminishes or accents certain parts of the wine's flavor), or by themselves. Don't spend lots of money on wine, the "sweet" spot to me for good wines tends to be between $7-$14 per 750ml at the store (the exact same wines will be about 3x that at a restaurant). Costco seems to be about the cheapest place to buy good wines, a $10 wine there is consistently $12-15 elsewhere. If you go to a winery, expect to pay around $20-30 for a bottle. Find what you like and TADA! You've found a good wine.

There are essentially two kinds of wineries, ones that grow their own grapes, and ones that buy grapes from other farms and ferment the wine themselves. Most wineries will also buy wines from other local wineries for their blends. There's nothing wrong with it and it's just how winemaking works.

Wine drinking is an acquired taste, many wines might be harsh or unpleasant on first sip, and by the bottom of the glass sublime. The taste might change a bit as you work your way through a bottle. When you read that wine has "hints of spice and plums" or whatever, it's just a way of describing the profile to you and what you might be able to expect. "Plum" might describe a deep fruity flavor (not sweet), "tobacco" describes some of the bitter tannins that come from the skins.

Wines also change flavor as they sit in storage. Some wines don't develop their full flavor profile until a couple years after they're bottled. Some wines are intended to be enjoyed almost immediately (Nouveaus).

Try wines from different places, the normal French styles are popular, but there are some incredibly good local varieties that aren't well known outside of their home countries (and are usually very cheap). Spain and Italy in particular have a rich and complex wine culture easily as great as France. Many popular varieties are also grown in places outside of France. California for example grows lots of traditional French varieties. Areas not necessarily known for good wine might actually mean that they don't have the right kind of soil or climate to grow these popular varieties. Virginia, for example, is too wet to grow really good Cabernets, but there are almost entirely unknown grapes bred just for Virginia that are excellent.

Wine glasses are intended for your nose rather than your tongue. They're designed as recognition that about half of our flavor comes from our noses, so different shapes of the glass are intended to help deliver vaporizing parts of the wine to your nose to help build up the flavor profile.

Wine drinking is not snobby, in places with a very long wine drinking history, wine is the normal table drink. It's nothing unusual to sit down in a reasonably priced family restaurant, order a plate of the local whatever and get a 500ml carafe of house wine as part of the order instead of water. It probably won't be great wine (might even be cut with a bit of water), but it'll go fine with the meal. In history, wine was the "safe" alternative to water, priests, construction workers, delivery men, soldiers, etc. all drank wine as part of their normal day-to-day.

Think nothing of sitting outside on your porch with a bottle of Merlot and a red plastic solo cup if you enjoy it.

A few of my personal favorites (all between $9-14 at Costco):

http://www.wine.com/v6/Penfolds-Koonunga-Hill-Shiraz-Caberne...

http://www.wine.com/v6/Santa-Rita-Reserva-Cabernet-Sauvignon...

http://www.winesu.com/zaccagnini-montepulciano.html

http://www.wine.com/v6/Excelsior-Cabernet-Sauvignon-2010/win... (about $6 at Costco with a twist off top!)