black tea is made from leaves that have been fully oxidized, producing a hearty deep rich flavor in a colored amber brew. It is the oxidation process, oxygen coming into contact with the enzymes in the tea leaf that distinguishes black teas from green. The oxidation process is also known as fermentation.

Assam (India):

Assam tea has distinctive flecked brown and gold leaves known as "orange" when dried. A fine, malty black blend with a rich, robust flavor from Assam, a major growing area covering the Brahmaputra valley, from the Himalayas to the Bay of Bengals.
The best leaves are picked during the second flush during the last week of May through the first two weeks of June. This tea is popular as a morning "wake-up" beverage because of its strength.

Ceylon (Sri Lanka):

There are several varieties of Ceylon tea, but most of the Sri Lankan harvest goes into blends. Commercial blends advertised as "Orange Pekoe" are usually blends of India and Ceylon.
This is probably closest to what most Westerners think of when they think of tea: reddish-brown liquor; brisk, full flavor.

Darjeeling (India):

This is the most expensive, sought-after black tea in the world. Unlike most other teas, many Darjeelings are sold under the name of the plantation where they were grown.
Unfortunately, a great deal of tea labeled 'Darjeeling' consists of blends containing only 50% Darjeeling. Worse, most of the Darjeeling contained in these blends is harvested during the rainy season and so is less flavorful. If you intend to buy real Darjeeling, make sure you are buying 100% Darjeeling, preferably first or second flush (see 2.7). Fine Darjeelings usually have a lighter liquor than other black teas, from a light reddish color to a bright gold. Astringency is usually quite pronounced, and the aroma and flavor hint of almonds and wildflowers.

Earl Grey:

Canada's favourite flavored blend, this tea was named for the second Earl Grey who was awarded the recipe from a Chinese tea-loving mandarin in 1830. Traditionally a blend of Chinese black tea, it is flavored with the oil of bergamot, a pear-shaped fruit grown in the Mediterranean. A very popular afternoon refresher.

Earl Grey (1764-1845) was an actual person who, though he was prime minister of England under Wiliam IV, is better remembered for the tea named after him. Tea legends say the blend was given to him by a Chinese Mandarin seeking to influence trade relations. A smoky tea with a hint of sweetness to it, it is served plain and is the second most popular tea in the world today. It is generally a blend of black teas and bergamot oil.

Keemun (China):

This is the foundation of many English breakfast blends. (Some English Breakfasts are all Keemun.) Keemuns come in a remarkably large number of varieties.
Most produce a red liquor with a subtle combination of flavors; the aroma is often rich and fruity, sometimes with suggestions of plum and apple. Some Keemuns have a delicate smoky flavor (though not as smoky as Lapsang Souchong).

Lapsang Souchong (China):

This tea is fired over smoking pine needles, which produces a striking smoky odor and flavor. The best varieties are not overwhelmed by the smoke, but retain subtlety and a mix of other flavors. Lapsang Souchong is found in many Russian Caravan blends.

Smooth and clean with a powerful smoky scent. Traditionally dried with smoke from burning pine root.

Many China teas have a subtle toasty note, but Lapsang Souchong has a dominating smoky flavor. Legend has it that the smoking process was discovered by accident. During the Qing dynasty, an army unit passing through a village camped out in its tea factory, which was filled with fresh leaves awaiting processing. When the soldiers left and the workers were able to return, there was not enough time to dry the tea the customary way. So they lit open fires of pine wood to hasten the drying. Upon taking the tea to market, to their surprise the new tea created a sensation. Today, Lapsang Souchong is first withered over pine root fires, then panfried, rolled and oxidized. The leaves are then placed in bamboo baskets and hung on wooden racks over smoking pine fires to dry and absorb the smoke. The result is a powerfully smoky aroma coupled with a sweet and mild taste.

Nilgiri (India):

Nilgiri, Darjeeling, and Assam are the three Indian teas which the Indian Tea Board promotes as "self-drinkers," i.e. teas worth drinking unblended.
Unfortunately, Nilgiri is not as distinctive or interesting as the other two. It is very much like Ceylon tea. Like Ceylon, much of the Nilgiri harvest ends up in blends.

Sikkim (India):

This variety comes from a tea-growing area very near Darjeeling. It combines Darjeeling's delicate flavor and light body with Assam's maltiness.
Although it is an excellent tea, it is not very well known (yet) and thus not quite as expensive as Darjeeling.

Yunnan (China):

Yunnan's brown liquor has a subtle, earthy, peppery flavor. Inexpensive Yunnan is not very exciting, but I am told that the higher quality harvests are wonderful.
Some Yunnan is used in Russian Caravan blends (see 1.9).

Popular blends:

English Breakfast
The exact blend depends on the dealer. Some English Breakfasts are blends of India and Ceylon teas; others, mostly or entirely Keemun.

  The prototype of this most popular of all teas was developed over a hundred years ago by the Scottish Tea Master Drysdale in EdInburgh. It was marketed simply as "Breakfast Tea". It became popular in England due to the craze Queen Victoria created for things Scottish (the summer home of Victoria and Albert was the Highland castle of Balmoral). Tea shops in London, however, changed the name and marketed it as "English Breakfast Tea". It is a blend of fine black teas, often including some Keemun tea. Many tea authorities suggest that the Keemun tea blended with milk creates a bouquet that reminds people of "toast hot from the oven" and maybe the original source for the name. It should be offered with milk or lemon. (One never serves lemon to a guest if they request milk-the lemon is never used. It would curdle the milk.) It may also be used to brew iced tea.

Irish Breakfast
As mentioned above, this is usually mostly Assam--and very strong.

The Irish have always been great tea drinkers, and they drink their tea brewed very strong. In fact, there is a common tea saying among the Irish that a "proper cup of tea" should be "strong enough for a mouse to trot on." Along the same line, the Irish believed there were only three types of tea fit to drink. The first and best of quality was in China with the Chinese, of course. The second best was sent directly to Ireland. The third and lowest in quality was sent to the English. Irish Breakfast because of its robust flavor is usually drunk only in the morning (except for the Irish who drink it all day). Usually it is blended from an Assam tea base. Because of its full taste, it is served with lots of sugar (loose is considered correct here-sugar cubes are an English matter) and milk (milk, NEVER CREAM, is served with tea. Cream is too heavy for tea and belongs with coffee. The milk is always served at room temperature, never cold, as it cools the tea too quickly).

Russian Caravan
A popular blend, Russian Caravan harks back to the days when tea was hauled to Russia from China on camelback. It often contains a bit of smoky Lapsang Souchong, though its base is typically Keemun or Yunnan. Many also contain oolong.