Dead languages are those which are no longer spoken. Some, such as Latin, are not strictly speaking dead – they have just evolved into other Romance languages. There was never a time when Latin stopped being spoken and Italian started; the language just slowly changed over time, much like ancient Greek becoming modern Greek. Sanskrit, an ancient language of India, has had a similar journey. It is maintained as the sacred language of Hindu worship, but just like Latin, it has evolved into other languages such as Hindi, Bengali and Gujarati.
Most languages that die, rather than evolve, do so when they are not passed on to the next generation for some reason. This is what happened to Gaulish and what is still happening too many languages around the world.
Probably the most famous of the dead languages is Egyptian, the language of ancient Egypt, with its distinctive hieroglyphic writing system. This language began its decline around 7 CE with the arrival of Arabic, following the Muslim conquest of Egypt.
Linguists now recognise the importance of keeping languages alive, and many languages at risk of extinction are given protected status. Languages such as Cornish have come perilously close to extinction but they have been revived and nurtured and now have a growing number of speakers.