Marble is a limestone or a dolomite that has recrystallized. Marbles are usually light in colour, but many exception to this generalisations are found. The presence of oxides of iron and graphite accounts for a wide range of colours and delicate markings.
Marble is known to scratch and stain and is therefore not the best choice for worktops in kitchen. It is often used in bathrooms mostly because the colour of the marbles lends itself of the more serene environment.
Cleaning is easy, just use soap and water, remember never to use ammonia based cleaning products as this will lessen the shine of the marble over time.