Dead languages don't change. This is why they can be useful, as is the case with Latin.

Latin is a very, very useful language because it doesn't change. That means it will be the same five hundred years from now. Because of this, scientists can say things in Latin, and have them be understood in the distant future.

Living Languages evolve, though.

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See below for interesting points.

That's not strictly true. Every year, there is a conference in the Vatican City State (where Latin is still the official national language) to determine what the Latin words for new words and ideas, like Coca-Cola or jet engine, are. Some of the results are quite amusing, but that's another story...

The only truly dead language is one which can't be understood anymore. All other languages are in various states of evolution.

There are various states to evolution, of course. Latin, and other languages people consider to be dead, such as Ancient Greek, have reached inertia. Any language, once its rules (grammar, spelling, etc.) are written down, will reach inertia. Any change in the language formed afterwards becomes a new form of the language that continues to evolve until it, too, reaches inertia.

Case in point: Victorian English.