In real life, of course a tragedy is any terrible event. In drama, especially classical drama, it's much more specific.
The dramatic definition of tragedy comes from the ancient Greek philosopher Aristotle. According to this definition, tragedy involves a man in a high position, such as a king, with great qualities, whose downfall is caused by his own errors (a "tragic flaw" in his own character.) Others are also brought down with him; tragedy is not private. The audience should be moved, not to tears, but to "catharsis", a Greek word meaning, literally "purging"; filled with pity and terror, and drained of less noble emotions. In other words, watching tragedy should make you, at least temporarily, a better person.
One example of classical tragedy is Sophocles' "Oedipus Rex". Oedipus becomes King of Thebes through his own intelligence, but eventually destroys himself, his family and his kingdom by his own actions; unwittingly murdering his father and committing incest with his mother.
To trace down the difference between tragedy, comedy and a tragi-comedy, we will first have to see what these terms mean. Tragedy is probably one the most debated issues in literature. No critic in the history has ever been able to define "tragedy" in an absolute way. Aristotle's definition of tragedy seems to fail now in modern times. The formative elements described by Aristotle have been rejected by many of the modern writers. For example, for Aristotle, Organic plot is the best suitable plot for a tragedy but in the modern and post modern era of literature we have seen a number of writers who have strongly disagreed with Aristotle. His definition has also been rejected. However, for the sake of a general understanding, we can say that tragedy is a play, which has an unhappy ending. It is not necessary that there must be some bloodshed at the end of the play. We can even have an unhappy ending without any kind of blood shed. On the contrary, a comedy is generally a play that has a happy ending and there are no losses in the play. A common understanding of comedy is that it is a play that brings smile to your faces. This is not a correct understanding of comedy. Thirdly, a tragi-comedy is a play that has a happy ending but we have some irreparable losses in the play. For example, in Shakespeare's "A Winter's Tale", we have a happy ending but the deaths of certain characters and golden time lost by the queen Hermione cannot be called back. Hence, it has got somewhat an element of tragedy in it but the ending is a happy one.
A tragedy is sad and a comedy is funny. Tragedy's are usually something that happens that causes sadness or grief. A comedy is something that is cognitively thought up and put into actions. Tragedy gives a feeling of sadness and comedy can be hilarious and funny giving pleasurable feelings.
I know that in the traditional sense, a tragedy is a play where one or more of the characters die (for example, Othello, Julius Caesar, Hamlet, Macbeth, Romeo and Juliet). A comedy doesn't necessarily have to be funny, it just means that all of the characters live to see the end of the play(Midsummer Night's Dream, Merchant of Venice, Comedy of Errors, All's Well That Ends Well).
That's the biggest difference that I know of. Hope this helps!
In purely literary terms Comedy is defined as stories that end positively, where the protagonist essentially triumphs over evil and all odds that he encounters in the story and gets rewarded in the end. It's the age old cliched and desired happy ending that the protagonist strives for and gains in the end of the story. Tragedy on the other hand deals with the human condition, it is essentially more rich than comedies as it seems to be reflective and brooding of our fallibility, weaknesses and focuses more on the melancholia that arises from existence. Tragic stories always end in tears and pain, but it is also more introspective and deep. They help us come to terms with the great deal of suffering we see around us, acknowledge it and strive to make it better.
