Popette is correct and has pretty much covered all three meanings.
As an adjective it means that something is sweet but slightly sour (agrio, acido) or sharp tasting, such as lemonade...the more sugar it has the less tart it is.
Tart as a noun is as Popette says, a pastry made with some type of fruit.
Tart as used to describe a woman is used more in the UK I believe, and is a woman of loose morals. It the U.S. we would more likely use the word "slut". (una fulana, ramera, prostituta, algo asi).
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