‘Tortie’ is short for tortoiseshell. ‘Torbie’ is a made up word which joins together ‘tortie’ and ‘tabby’ (the second ‘t’ becomes a ‘b’). It describes a cat coat which is tortoiseshell with tabby fur mixed in. There is an endless spectrum of combinations, some very subtle. It can be tricky deciding if a cat is a torbie or a tortie. The word ‘tortoiseshell’ describes the well-known orange and black cat coat and the word originates in the name of the material which comes from the shells of the larger species of tortoise and turtle.
Personally, I look for the most outstanding indication that a cat is a tabby cat: the ‘M mark’ on the forehead. If it is there and if the cat looks like a tortoiseshell, then the cat is a torbie. The M mark can occasionally be subtle and not obvious but it will still be there in a tabby cat; you just have to look for it. It is very difficult to see individual tabby hair strands in a torbie cat unless you brush your hand through the fur and inspect it closely.
Individual cat hair strands contain two types of color pigmentation: eumelanin (black/brown) and phaeomelanin (red/yellow pigment). The tortie has both in solid color in varying densities and places throughout the coat. The torbie has both colors as well but the agouti gene breaks up the pigmentation on the hair strands – there is a banding in the hair strand as opposed to solid color.
You can get tortie-and-whites (calico) and torbie-an-whites too.