Tea is something we think of primarily as being a drink, rather than a seasoning. But there are many ways that tea leaves can be used to infuse ingredients. Tea leaves come from the tea plant, Camellia sinensis (if a drink is made solely of leaves or flowers from another type of plant, it's called a tisane). The curing process, which can include drying, baking, fermenting, blending and shaping, determines the colour and type of tea, which is then brewed with hot (or with some types, boiling) water.
Probably the most common way to use tea leaves as flavouring is through smoke. It's usually a little more complicated than simply throwing tea leaves onto a charcoal or wood fire, though - you can do that, but the flavour would be, at best, elusive - because the dried leaves would burn quickly and wouldn't give off much smoke. Usually, tea leaves are mixed with other ingredients that smoulder, rather than quickly catch alight, such as sugar, cinnamon bark, star anise, peppercorns and uncooked rice.
Another way to get a tea flavour is by using the leaves as part of a brine. A brine is usually made by mixing salt, sugar and other ingredients into water, in which meats are then soaked; by osmosis, the relatively dense brine is sucked into the meat, flavouring it. Adding tea leaves to the brine adds another flavour dimension, although it should be done judiciously, so it complements, rather than overwhelms, the other seasonings.
In desserts, a very tiny amount of powdered green tea can be added to cake and cookie batters, to give them a pretty green tint and a subtle flavour. Don't use too much, though, or the dessert will be bitter. You can also infuse tea (leaves or bags) in hot cream, then use this to make a ganache.