Tea are the leaves of one specific plant, Camellia sinensis.
Strictly speaking, if you poured boiling water over the dried leaves of any other plant, what you would have is an “infusion,” not tea, although such concoctions are often called “teas,” e.g. chamomile tea. You could certainly try it with any old leaf off the ground, but it’s unlikely to taste very good.
In addition to being from a specific plant, actual-tea is processed differently to make different kinds of product.
If you just take the whole leaves and try them, you get white tea which has a very delicate (some might say almost nonexistent) flavour.
Most of the interesting-flavoured compounds in tea are produced by oxidising and/or fermenting the leaves before drying them.
If you just oxidise them a little bit (by mildly bruising them and allowing them to sit a while before drying), you get green tea – the leaves are still green, but you get a greeny-yellow liquid from steeping them in water, with a mild flavour.
If you chop them up and leave them to sit longer before drying, the air and the enzymes really get to work transforming the chemical composition of the leaf juices, and you get black tea – producing a red-brown liquid with a lot more astringent tannins and a deeper flavour.
ETA: also the type (eg young leaves from the tips vs older leaves) and the exact variety, growing conditions, etc all go into the differences between different teas.
Plus some are heat treated, some are cultured with fungus, etc. etc. it’s as intricate as winemaking and cheesemaking, seriously.
What the other comments didn’t mention is that actual tea normally contains caffeine (same as coffee) and for this reason has a (mild) stimulating effect. The extent of this effect depends on how strong you make your tea. Herbal infusions from some other random leaves are unlikely to contain any caffeine.
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