There are a few different types of opals available.
Some of the more common are:
The Black Opal has a dark body tone which causes a brightness of color unmatched by the other opal types. Black Opals are only mined at Lightning Ridge in New South Wales in Australia and considered very rare and are consequently very expensive
See Black Opals for more information.
White Opals, by contrast are almost cream and sometimes called a Milky Opal
Being white they do not show the internal color as well as the black and are generally purchased as white opals only. In an excellent quality stone however there can still be seen some color flashing.
More information can be found at White Opals.
Boulder Opals are different again.
Bounder opals have a solid layer of brown ironstone on the back of the stone. They originate from large ironstone bounders underground hence the name. More information about Boulder Opals is available here
A Crystal opal can be the same as above but has a transparent or semi-transparent body tone which means you can see through the stone. Crystal opal can have a dark or light body tone hence, "black crystal opal" and "white crystal opal"
A Fire Opal is an American term and describes any opal which has a significant amount of red colouring. Red is the rarest colour in opals, so these are quite valuable. Often confused with a 'Mexican fire opal' which is different yet again and displays orange instead of red coloring.
A Matrix opal is where the opal occurs as a network of veins or infilling of voids or between grains of the host rock. A Matrix comprises precious opaline silica as an infilling of pore spaces in silty claystone or ironstone. the matrix opal shows a fine pinfire colour in the natural state. very rare also