The City of Venice in northern Italy was known in Renaissance times as 'La Serenissima' ('the most serene') and was a wonder of the world at that time, being home to the finest architects, artists, sculptors and thinkers of the day -- as well as practicioners of a different kind such as Casanova. It still retains much of its ancient charm although it has pressing modern problems with increased flooding due to sea level rise and a shrinking native population caused by the absence of jobs and rising costs. Wikipedia can give you the full history if you're interested.

The Venice Carnival is, essentially, an annual Mardi Gras/Lent/Ramadan/... festival but with a very long history. There are more activities available than you can possibly attend (balls, banquets, canal parades, ...) but the main attraction is the costumes and masks that people wear for the event. Some of the revellers are paid performers but most are people from all over the world simply joining in with everything from the enormously elaborate (this lady won the costume contest in 2013):

To the mostly humorous:

To the slightly creepy:

Wherever you go in Venice during Carnevale you will run into people in costume. Sometimes it feels like you're an outsider when you're not in costume (I operated in both modes and had a great time either way!)

Venice is actually a large enough city that you can get away from the main centers of activity and just enjoy Venice the way it has been for centuries:

And when Carnevale is over, it really is over. This shot was St Marks Square (the one you always see on TV or in the news) the evening after the official end of the festivities. Where did all those people go?!

There are some more images in the Venice gallery here.