A blackboard, with multiple colors of chalk
A blackboard is a surface on which markings made with chalk are visible. It is used as a surface on which to write.
A blackboard can simply be a piece of board painted with matte dark paint (usually dark green or black), but a more modern variation consists of a looped sheet of plastic on rollers, which can be scrolled to reveal more blank writing space.
Blackboards are often used in teaching. The chalk marks can be quickly and easily wiped off with a damp cloth, or a special blackboard eraser, a block of wood with a felt pad. Somewhat indelible chalkmarks can be made on a blackboard when the board is wet.
Sticks of processed "chalk" are made especially for use with blackboards, in white and also in various colours. These are not actually made from chalk rock, but from gypsum.
Blackboards have disadvantages: they produce a fair amount of dust, depending on the quality of chalk used. Some people find this uncomfortable, or may be allergic to it, and there has been speculation about links between chalk dust and respiratory problems. However, other methods of displaying information are more expensive and have drawbacks of their own. These disadvantages have led to the widespread adoption of the whiteboard, which uses ink markers that produce no dust.
The scratching of fingernails on a blackboard is a sound that is well-known for being extremely irritating.
Blackboard (http://www.blackboard.com) is also the name of a company that provides software for online education.