I know from this question that doubles and triples are stronger than the "single" version. And from what I've seen, Belgian triple are often closer to be blond and golden while doubles are often darker.
My question though is what should I expect from a triple? What are it's defining characteristics (meaning that I can recognize one when I drink it, or recognize an off-style one)?
I expect a high alcohol beer, with a lot of complex flavors. One that the alcohol content will "sneak" up on you because it isn't very obvious in the taste.
Quoting from the Beer Judge Criteria:
Aroma: Complex with moderate to significant spiciness, moderate fruity esters and low alcohol and hop aromas. Generous spicy, peppery, sometimes clove-like phenols. Esters are often reminiscent of citrus fruits such as oranges, but may sometimes have a slight banana character. A low yet distinctive spicy, floral, sometimes perfumy hop character is usually found. Alcohols are soft, spicy and low in intensity. No hot alcohol or solventy aromas. The malt character is light. No diacetyl.
Appearance: Deep yellow to deep gold in color. Good clarity. Effervescent. Long-lasting, creamy, rocky, white head resulting in characteristic “Belgian lace” on the glass as it fades.
Flavor: Marriage of spicy, fruity and alcohol flavors supported by a soft malt character. Low to moderate phenols are peppery in character. Esters are reminiscent of citrus fruit such as orange or sometimes lemon. A low to moderate spicy hop character is usually found. Alcohols are soft, spicy, often a bit sweet and low in intensity. Bitterness is typically medium to high from a combination of hop bitterness and yeast-produced phenolics. Substantial carbonation and bitterness lends a dry finish with a moderately bitter aftertaste. No diacetyl.
Mouthfeel: Medium-light to medium body, although lighter than the substantial gravity would suggest (thanks to sugar and high carbonation). High alcohol content adds a pleasant creaminess but little to no obvious warming sensation. No hot alcohol or solventy character. Always effervescent. Never astringent.
Overall Impression: Strongly resembles a Strong Golden Ale but slightly darker and somewhat fuller-bodied. Usually has a more rounded malt flavor but should not be sweet.
Comments: High in alcohol but does not taste strongly of alcohol. The best examples are sneaky, not obvious. High carbonation and attenuation helps to bring out the many flavors and to increase the perception of a dry finish. Most Trappist versions have at least 30 IBUs and are very dry. Traditionally bottle-conditioned (“refermented in the bottle”).
History: Originally popularized by the Trappist monastery at Westmalle.
Ingredients: The light color and relatively light body for a beer of this strength are the result of using Pilsner malt and up to 20% white sugar. Noble hops or Styrian Goldings are commonly used. Belgian yeast strains are used – those that produce fruity esters, spicy phenolics and higher alcohols – often aided by slightly warmer fermentation temperatures. Spice additions are generally not traditional, and if used, should not be recognizable as such. Fairly soft water.
I am a big fan of tripels and quadrupels. They are dark, and much smoother, I find, than blondes and other lower-alcohol beers (and yes, tripels are substantially more alcoholic). Contrary to the previous answer, I find they taste less bitter than typical beers, although perhaps it may be because the bitterness is masked by the other flavors perhaps? It may depend, though; Chimay yellow label has a note of bitterness that I don't find in, say, a Westmalle.
Part of it depends on what you're used to. If you're used to mass-market beers you will find these very different. Heavier and more substantial. Richer, more luxurious. Very much not something you quaff down a few at a time to quench your thirst; these are savored.
There are also quadrupels, such as the Roquefort 10 or Westvleteren XII, which are even better in my mind. These are very dark and pack an alcoholic punch.
Depends what you mean with Belgian Triple. As I understand in the US it is used as a style name and it is base on the Westmalle Triple. You can see the style discription in the answer of Schleis. But if you want to know what to expect, try Westmalle Triple, it's a good (my preferred) benchmark.
In Belgium it's not a style. It's merely a label to mark that more ingredient were used, comparatively with the other beers within the same brewery. A little history: The ingredients to make beer were expensive in the middle ages, so the brewers tried to use as little as possible for there beer. There was however also an market for more tasteful, more expensive, beer. To differentiate between the brewed barrels they used X for the cheap bear, XX for the expensive one. Not saying that there were double the ingredients in it, just more. Later came also XXX. You can still find these X's on some labels these days. The name Triple is a more recent appellation (1930's), but it is based on the same idea. A classic trio you find in Belgium, but with a lot of exceptions, is a blond(X), a brown (XX, double) and a strong blond ale (XXX, triple). These are relative references, within the brewery and like already mentioned, more ingredients, not doubling or tripling the amount of ingredients. Making the beers stronger in alcohol and taste.
I guess you now also know what to expect with a quadruple...
As a Belgian chap, I obviously love our beer :). More specific, my favorite beers are always tripel beers. The thing about tripel beers is that the fermentation is achieved further in the bottle.
In general I can tell you all tripel beers are always pretty blonde looking (maybe slightly darker) and doubles are always darker (like really brown!). In Belgium, we would call a double beer a dark beer (Double Leffe we would call: Donkere Leffe or Dark Leffe in English).
The taste of double beers will always be sweeter while tripel beers are far more bitter. I don't like those sweet drinks so I very much prefer tripel beers. However, all tripel beers are different from each other so there is not a real way of describing them. In general bitter, but there are sweeter tripels as well...
The difference in taste between blonde and tripel beers in general is that tripel beers have far more flavors (= more expensive) than blonde beers. The sad thing is that blonde beers are very popular and I don't even know why... Tripel beers have way more flavors :).
As a side note, my favorite beers are: Tripel Karmeliet and Tripel Kasteelbier. The more poular (blonde) beers are Duvel and Leffe for example. Both in my opinion hugely overrated beers.
As stated above, if you want to know about a certain style the BJCP guidelines are a great place to start.
Not all styles are reflected in the BJCP guidelines though. For instance, the BJCP doesn't recognize Black IPAs, or American Wild Ales. The Beer Advocate style guidelines, and the guidelines from the Great American Beer Festival might also be useful to you.
With regard to your specific question about Tripels, you can read my own thoughts about them on my blog (I'm trying to drink all the BJCP styles are write about them):
