There are many ways to take tea. The English traditionally drink their tea with milk and often sugar, while the Indians add to this a range of spices; the Moroccans and other north Africans take sweetened green tea with mint, while the inhabitants of east Asia mostly take the same kind of tea plain. In Russia, black tea is traditionally drunk with lemon and sugar, or sometimes on its own, or with jam. Tea first reached Russia from China in the early days of tea-drinking - back when tea came pressed into bricks which were boiled into a soup, with salt, spices and fruit like lemons; the Chinese long ago fell out of the habit of taking fruit with their tea, but in Russia the practice never died out.
Adding a squeeze of lemon juice to black tea has the effect of clearing the liquid; it transforms it from a dark, nearly-opaque brown to a transparent orangey yellow in a matter of seconds. This is because the hydogen ions produced by the acid in the lemon juice suppress the ionisation of thearubigins (tannins), the polyphenols that otherwise give tea its brown colour, an effect known as the bathochromic shift - it's a fun chemistry demonstration as well as a tasty hot drink!* The reaction makes the thearubigins lose not just their brown colour but also their astringency, so strong black tea can be made drinkable this way - especially with the addition of a little sugar to take the edge off the bitterness. Unlike milky tea, lemon tea made this way remains tasty at any temperature, and many recipes for iced tea call for the addition of lemon. Incidentally, orange juice is also acid enough to remove most of black tea's astringency, as well as sweetening it; it sounds weird, but it's actually pretty good.
Another kind of lemon tea is made in the same way as Earl Grey (which is flavoured using the bergamot, a kind of orange): black tea or green tea is flavoured with essential oils taken from the peel of the fruit, where the aromatic compounds are concentrated. Lady Grey tea is Earl Grey with added lemon and orange scents. Lemon is also used in various kinds of herbal tea; I have seen it added to chamomile, lemongrass and ginger, among other things. Lemon and honey is not quite a tea, but it is such a good thing to drink when you have a cold that I feel it is worth mentioning all the same.
There is some evidence that drinking lemon tea significantly reduces the risk of skin cancer†. The chances are that it also helps to counter a whole range of other cancers, as well as heart disease, because tea in general is very rich in antioxidant polyphenols. A 2001 study strongly suggested that drinking tea with milk destroys the benefits of these chemicals; they seem to be mopped up before they ever reach the blood, so if you want to drink strong tea which has plenty of caffeine, isn't mouth-shrinkingly astringent and still gives you the full health benefits of tea, lemon tea may well be the way to go.