Chalk is formed in a deep marine environment from the shells of microscopic algae which flourish near the surface, but die and fall to the seafloor. The kind of chalk used in old-style classrooms and lecture theatres scrapes off a thin layer of fine white dust as they are scraped over the abrasive surface of a "blackboard".
Limestone is formed in a shallower marine environment from the bodies of corals and shells, as well as the shells of microscopic sea life. Since it contains large lumps, it does not form such a fine white powder as chalk.
Marble is metamorphosed limestone, ie subjected to heat and pressure, the crystals grow and interlock, forming a stronger rock, more suitable for a building material. Sometimes minerals in the original limestone are still visible as swirls of colour in the marble.
[1] http://www.thenakedscientists.com/forum/index.php?topic=49775.0
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