Chalkboards gained popularity quickly after their introduction to classrooms in 1801. People today take for granted that there are enough pencils for students, that copies can be made for handouts and that teachers can display an example of something to the entire classroom. This was not the case in the past, and the chalkboard was a great leap forward in mass instruction. Teachers could easily display examples and instructions and just as easily erase and replace them. Eventually, each student was provided with a "slate" upon which to practice his work.
Initially, chalkboards were called blackboards, because the most common material for chalkboards was black slate. Slate was plentiful and easy to work with, before the introduction of more modern, paint-on laminates with matte (rough) finishes. The purpose of a chalkboard is to provide a reusable surface. Chalk wipes away easily, so that surface needs to be fairly impermeable in order to clean it with a damp rag or a felt eraser.
Chalkboards can now be made with any impermeable surface---like plastic or wood---that will adhere to a coat of paint. The highest quality chalkboards are made with a sheet of steel, coated with porcelain enamel. Newer chalkboards are most commonly green, because green is considered easier on the eyes than black. Some paint manufacturers now make paints specifically for turning surfaces into chalkboards.
Chalkboards have largely been replaced in modern classrooms by dry erase boards and electronic audio-visual aids. However, chalkboards remain in some elementary school settings for the children to use, because many educators believe that chalk is an easier medium for small children to work with than pencil and paper. Chalkboards are also still commonly used in restaurants and other places where daily announcements---like menu specials or upcoming events---can be displayed and changed.