Problem with mysql2

I have a problem with mysql2 and "Library not loaded" when trying to
access a rails app. What could be wrong? I've searched the net and found
others with similar problem. But not sure what the answer is.

What if I use the old gem mysql instead? What's the difference?

A uninstall and reinstall of mysql2 solved it.

Still, could someone list the difference between the old mysql gem and new mysql2 gem??
Why use the new one??