Marble, like granite, is a natural stone.  Unlike granite, it is a sedimentary, metamorphic stone which started as limestone and, once superheated, turns into marble.  It is a soft stone which is very susceptible to scratches, staining and etching (caused by anything acidic).  However, because of it's beauty and classic look, some customers will still choose marble, with the full knowledge that character will form over time. Marble is a porous material and will be sealed with a 10 year impregnating sealer prior to install.  Sealers will help protect stone against staining, but they do not protect stone from etching.  It is extremely heat tolerant.  

There are many different marble colours, ranging from white to green to black.  There are a few options pictured here, but there are many other colours available and each slab will be unique.