English Breakfast is a popular style of tea both in the UK and in the US. In the UK, it is referred to simply as "tea". I say "style of tea" and not "type" or "variety", because English Breakfast is defined not by the particular teas used in it, but by the overall character of the tea: how it tastes.
Typically, English Breakfast is a blended tea, produced by blending black teas from different regions of the country. The blended nature of this tea is due partly to cost, as blending can save money as tea prices fluctuate in different regions of the world, but it also helps to maintain a consistent flavor.
What teas are used in English Breakfast blends?
The most common teas used in English Breakfast are Ceylon and Assam. Kenyan teas are also popular. Keemun is slightly less common, but is still sometimes used.
Ceylon: Black Tea from Sri Lanka
"Ceylon" is just an archaic term for Sri Lanka, the small island country just south of India, in the Indian ocean. Ceylon tea can be quite diverse, but it is the type of tea that tends to be closest in character to English Breakfast itself. Many British teas are made exclusively from Ceylon, and many single-origin Ceylon teas have a distinctly British character to them.
Assam: A Strong, Robust Black Tea from Northeast India
Assam has a reputation in the tea world as one of the strongest black teas. It tends to be robust and full-bodied, and has an aroma that is often described as malty. Assam packs a punch, and as such is a favorite ingredient in strong breakfast teas. English breakfast, however, tends to be a little milder than Irish Breakfast blends, so the proportion of Assam in English breakfast blends tends to be slightly lower. The greater the portion of Assam, typically, the stronger tasting the blend.
Other Teas: Kenyan Black Teas and Keemun
Kenya is a major tea producer, but until recently, it was not well-known for single-origin teas. This is quickly changing. Kenyan black tea has a reputation for being strong like Assam tea, but it often shares many of the flavor and aroma characteristics of high-grown teas like those from Darjeeling, India. Kenyan teas have been a major ingredient in English Breakfast blends.
Keemun is a very different sort of tea. It is a Chinese black tea, with more of a warming quality than the other teas discussed above. Keemun is smooth-tasting and fruity, but rich and full tasting. It sometimes has a slight smokiness to it, and tones of fruit, sometimes described as suggestive of Burgundy wine. Keemun was a prime ingredient in old-fashioned English Breakfast teas, although it has fallen out of favor in more modern blends. A few companies however include Keemun as one ingredient or even a main ingredient in their blends, if they're looking to capture the character of these old-style teas.
English breakfast is defined primarily by its character, not the specific types or varieties of teas used to blend it. It typically contains Ceylon and Assam teas, and often teas from Kenya, and historically it was often made from Keemun. If you want further information on English Breakfast teas, you can find listing specific reviews on RateTea's page on English Breakfast tea; this is a site where I am both an editor and reviewer, and I have reviewed over a dozen English breakfast blends.