Latin is the term for the spoken language of Ancient Rome. Classed as a 'dead language' in that it is not spoken by any nation as their natural tongue (although it remains the official language of the Vatican City), its use is retained in the legal and medical professions, and it is also used extensively in anatomy and physiology, botany and other 'natural sciences' for the classification of animals, birds, fishes etc. It also finds favour amongst the Catholic chuch, enabling users thereof to pass smutty jokes without alerting Joe Public.
It has recently found favour among poseurs who drop the odd Latin phrase into conversation to demonstrate their intellectual superiority, and invariably proves useful shortly after to explain to paramedics that the speaker is suffering from a broken ethmoid bone and that coagulation is normal.
Some famous Latin phrases:
Cogito Ergo Sum = I think, therefore I am
Veni, Vidi, Vici = I came, I saw, I conquered
Veni, Vermini, Vomui = I came, got ratted, threw up
More useful Latin phrases in this ARRSE thread.