What is an elegy?What points are necessary to make a detailed note on elegy?

An elegy is a mournful, melancholic poem, usually written
as a lament for someone who has died.

In ancient Greek
poetry, the term elegy referred to a very specific form of poetry. 
Although Greek elegies could be about a wide variety of topics, they had to be written
in elegaic couplets, meaning in pairs of lines that had a very specific pattern of
stressed and unstressed syllables.

In later times, the word
elegy came to mean that the topic of the poem was mournful, even
the form of the poem might be completely unlike the ancient Greek
elegaic couplets.

One of the most famous elegies in the
English language is Thomas Gray's "Elegy Written in a Country Churchyard," in which the
poet reflects on the lives of the simple people who are buried in a rural churchyard. 
Although the poem clearly is elegaic in content, it does not use
any of the technical forms of the ancient Greek elegies.