Traditionally this is a blend of China teas or China and Indian tea scented with oil from the citrus bergamot fruit (a sort of orange). There are quite a lot of story about the name of this tea comes from.
One tale tells how a British diplomat on a mission to China, saved the life of a Mandarin and was given the recipe for this flavoured tea as a token of thanks, and as a gift to the then British Prime Minister, Earl Grey (1830-1834). Another legend says it was the Earl himself who saved the Mandarin and was given the recipe.
The name and stories may just have been a clever marketing ploy by whoever created this scented mixture.
Certainly, Earl Grey tea is very popular today. The amount of bergamot varies, and this makes an important difference to how good the tea will be.