Dubbels are sort of the little brothers to Quads, which are higher octane and use more malt, thus have more fullness. Both are typically dark, complex Belgian ales with pruney, rich, stewed stone fruit and spice notes.
Tripels are paler in color with an alcohol content in between, and usually accentuate more flavors reminiscent of banana and clove, and/or stone fruits.
Per Beer Advocate:
The Belgian Dubbel is a rich malty beer with some spicy / phenolic and mild alcoholic characteristics. Not as much fruitiness as the Belgian Strong Dark Ale but some dark fruit aromas and flavors may be present. Mild hop bitterness with no lingering hop flavors. It may show traits of a steely caramel flavor from the use of crystal malt or dark candy sugar. Look for a medium to full body with an expressive carbonation.
(abv) range: 6.5-9.0%
The name Tripel actually stems from part of the brewing process, in which brewers use up to three times the amount of malt than a standard Trappist "Single." Traditionally, Tripels are bright yellow to gold in color, which is a shade or two darker than the average Pilsener. Head should be big, dense and creamy. Aroma and flavor runs along complex, spicy phenolic, powdery yeast, fruity/estery with a sweet finish. Sweetness comes from both the pale malts and the higher alcohol. Bitterness is up there for a beer with such a light body for its strength, but at times is barely perceived amongst the even balance of malts and hops. The lighter body comes from the use of Belgian candy sugar (up to 25% sucrose), which not only lightens the body, but also adds complex alcoholic aromas and flavors. Small amounts of spices are sometimes added as well. Tripels are actually notoriously alcoholic, yet the best crafted ones hide this character quite evil-like and deceivingly, making them sipping beers.
(abv) range: 8.0-12.0%
Inspired by the Trappist brewers of Belgium, a Quadrupel is a Belgian style ale of great strength with bolder flavor compared to its Dubbel and Tripel sister styles. Typically a dark creation that ranges within the deep red, brown and garnet hues. Full bodied with a rich malty palate. Phenols are usually at a moderate level. Sweet with a low bitterness yet a well perceived alcohol. Quads can also go by the name BSDA, or Belgian Strong Dark Ale.
(abv) range: 9.0-13.0%