"Una Furtiva Lagrima" is the romantic aria (aria is a music piece for one voice, usually part of a larger work) from act 2 of the Italian Opera "L'Elisir d'Amore", composed by Gaetano Donizetti. The opera is considered a comic opera since it has romance as its central plot but also a happy ending.
The aria is sung by the character Nemorino (tenor), when he realises that the love potion (actually a cheap red wine) that he bought to win the heart of his dream lady, actually works and that she finally responds to his passion. He understands this by seeing a tear in her eye (the name of the aria translates to "a secret tear").
This is probably Donizetti's most famous work, which became widely known in the recent years through Luciano Pavarotti's great vocal skills. It is an unusual aria in the sense that although the melody is sad, the lyrics are joyfully expressing Nemorino's gratitude and immense happiness. Despite this incongruence, the aria is very popular with a captivating melody.