Tires: Confusion About Specificity of Tire Sizes, tirerack com, mustang v6

Question
Hello, I'm currently driving a 2005 Mustang V6. I need to buy new tires because the traction on the ones I currently have is not the best anymore. The size of each tire should be P215/65R16 according to the placard. I have two questions. 1. What do these numbers actually mean? And the second is, I want to buy specific tires, but they do not have the specific size my mustang needs. Can I use a different size and still be able to have regular functionality with them? The sizes they offer with respect to the rim size are: 205/45R16, 225/50R16, 205/50R16, 225/55R16, 215/55R16, and 205/55R16.

Thank you for helping.

Answer
Esteban,

Tire Rack has done a good job of explaining tire sizing:

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=46

I did a quick look up and allow me to cut to the chase:  

NONE of the tire sizes you listed is appropriate for your vehicle, so you need to be doing some more research.  The size you mention is popular enough that you should have a variety of choices.