The inner core is solid, the outer core fluid and extremely hot. Movement in the outer core is thought to be resonsible for the generation of the Earth's magnetic field. The mantle is heated from inside, which creates convection currents. The 0 - 100 km thick crust floats on the mantle and carries the continental plates.
Southampton Oceanography Centre, Challenger Division for Seafloor Processes (2004) Classroom@Sea Project. http://www.soc.soton.ac.uk/CHD/classroom@sea/carlsberg/images/plate_history_lge.jpg (accessed 22 August 2004). Image based on artwork from the US Geological Survey.