Her "breed" would be Domestic Shorthair (which is basically the feline equivalent of "mutt") since most housecats are so mixed determining a specific breed can be so hard to do.

Her markings, however, look calico from the picture you've described, since in order to be a "true" tortie you can not have any white. Do you have any other pictures of her to see her markings better?

http://www.thecatsite.com/general/colors.html

Check out that link for more information on coat patterns and colors