http://i.imgur.com/uyXIM.jpg
A bicolor domestic. In other words, a cat of no particular breed just like over 97% of the cats on this entire planet.
Cats were domesticated thousands of years ago and breeds only established in the last hundred or so years. So most cats have NO breeds whatsoever in their history. Add to that the fact breeders tend to sell their kittens altered and never allow crossbreeding so you just don 't end up with "mixes" like so many people incorrectly think.
LOL at the 3/4 domestic/1/4 Siamese answer. Where'd you pull that out of?
Looks like a pretty Black and White Domestic Longhair. Striking markings.
I have no idea where that other answerer saw any Siamese in her!
When it comes to cats and breeds, the first thing to understand is that cats are not like dogs where most are some breed or mix of a couple breeds that a knowledgeable person can usually recognize. There has been far less controlled cat breeding over the years than dog breeding.The vast majority of cats have just bred randomly on their own and they have a mix of genes from the general cat population in their part of the world. They come in a big variety of colours and patterns, and they're referred to as "Domestic Shorthairs" or "Domestic Longhairs" which just means no particular breed/ancestry unknown.
i'd say a normal domestic or 3/4 domestic and 1/4 siamese.
If you got him/her at a shelter I would say s/he is a mixed breed.