Dark chocolate contains 55% to 100% of cocoa solids. It is dark brown, very intense, and compliments everything, from raspberry to rice with nori.
Chocolate is a complex and bioactive product (it contains caffeine, theobromine and several other alkaloids), much like coffee, wine, tea or cheese. Like wine or coffee, it's not just tasty, it's mildly mind-altering. But unlike wine or coffee, chocolate bar is something made only by the modern technology (Meso-Americans who domesticated cacao tree and thought cocoa to be the food of gods used to drink it).
Eating chocolate comes in dark, bittersweet, milk and white varieties. Dark is 55% to 100% of cocoa solids with sugar and sometimes vanilla added. Bittersweet is 50%-60%, contains milk solids as well. Milk chocolate has even more milk and little cocoa -- 10% to 35%, and sometimes high-fructose syrup instead of sugar. White chocolate is no chocolate at all: it has no cocoa and sometimes even no cocoa butter, it's just sold in bars wrapped in foil and paper as chocolate proper.
Chocolate is made of cocoa beans. Cocoa grows in tropical habitats, in Africa, Asia, Hawaii and Southern America. Fruits of cocoa trees are fist-sized and almond-shaped. They are harvested, cut across and fermented on the spot. Fermented beans are dried and transported to the factory, there they are roasted and hulled and crushed into small pieces called cocoa nibs. Nibs from different batches are tasted and combined to achieve the desired taste, then ground into thick mass called cocoa mass or cocoa liquor. Cocoa butter is extracted and the remaining defattened liquor is pulverized to make cocoa powder. Chocolate bars are made of chocolate liquor with chocolate butter and sometimes cocoa added to improve flavor and texture. Eating chocolate is tempered, which means crystallized in particular way (beta-crystals) to make it shine, snap when broken, and melt less easily.
There are different brands of chocolates at different price ranges, and I admit I never tried the fanciest. Chocolate is fashionable nowdays and marketed for its healthfulness (particularly for women with PMS), sustainability or exotic source. Organic single-source chocolates in medium price range ($2-$5 for a 3.5 ounce bar) were a disappointment. At typical 70% of cocoa they are too sweet to tell the nuances of taste, but they clearly taste "something", usually sour or bitter-sour, like over-roasted coffee. The chocolate technology didn't learn the right way to make single-source chocolates yet. But it improved much in last couple of years, so there is hope.
The best dark chocolate I know of so far is Lindt Excellence 85% cocoa. The sweetness is just enough to bring out the chocolate flavor, but doesn't drawn it, and the taste itself is well-balanced, not bitter, or burnt, or sour as it happens with chocolates sometimes. The 90% chocolates taste a bit flat and 100% are plain bitter. I use 70% and even some 55% for my chocolate truffle filling, but 85% for coating. 70% is more forgiving than stronger one, and several other brands work well for ganache as well. Organic Whole Foods Dark Chocolate is bit plain, but OK with addition of cognac. Trader Joe's Belgian Dark Chocolate is 72%, comes as generic "Belgian" in 10 pounds slabs and it's surprisingly good. It has very mild taste, and judging by the way it melts and mixes I believe it has added cocoa butter. Hershey Dark is dusty and somewhat salty. Truffles and musse made with it taste bland and store-made. Ghirardelli is better, but too bitter despite all sugar. Scharffen Berger is very good, but expensive. Same for Black&Green: it's twice the price of Lindt and not that much of an improvement.
Dark chocolate goes well with tea, coffee, cognac and certain wines, but also is a great addition to sauces.
See also: kitchen, chocolate truffles, chocolate mousse.