Tortoise shell (or "torties") are cats with a mixed black and red fur with some times some white. Some are called "dilute" when the black and red are replaced by "blue" (light grey) and beige.

Calicos are cats whose fur shows large patches of these three colors.

Some genetics

Tortoise shell color is due to the inactivation of one of the two X chromosomes in a given region. Hence, you can't have male tortoise shell cat as he has only one X chromosome. Nevertheless, one case allows "male" torties is hermaphrodism (XXY). As these cats bear 3 chromosomes among which are 2 Xs, he can have a tortie fur. This case is extremely rare and these cats are sterile.

What difference is the between a tortie and a calico ?

The calico cat owns the "tortoise shell" genotype AND an additional genetic determiner producing white patches .

Against common thoughts, the placement of the patches or colors doesn't change in time. They will stay in the same place during all the life of the cat.

More on the subject here : http://en.wikipedia.org/wiki/Tortoiseshell_cat

. Tortoise shell are cats where black and red hair are mixed, calicos show large white patches.

. In most cases they are females.