During World War II, a successful music teacher named Harold Rhodes built a keyboard to entertain wounded servicemen. According to his "New York Times" obituary, it was "a 29-note keyboard using aluminum tubing from a B-17 to make a xylophone-like instrument, called the Army Air Corps lap model piano." Rhodes eventually developed a standard 88-key electric piano, which was sold to Japanese synthesizer company Roland in 1987. Concurrently, Yamaha Pianos had developed an electronic grand piano. Yamaha began to sell digital keyboards in the mid 1980s, and as of 2010 it remains one of the dominant makers of the digital piano.
The 1983 public release of the MIDI ("musical instrument digital interface") protocol made it possible to manufacture "digital" keyboards small and inexpensive enough for individual home use. The original MIDI specifications encoded digitized information about the "attributes" of musical notes (e.g., pitch and length), thus allowing closer approximation of acoustic notes. By the early 1990s, digitization allowed for hundreds of instruments and sound effects to be replicated on a single keyboard.
Digital keyboards in 2010 can cost anywhere between a couple of hundred and several thousand dollars, depending on their physical size, number of keys and gadgets, and approximation to the sound and feel of a good traditional piano. "Digital pianos" tend to be more expensive and have fewer functions; they're generally used by people who want an acoustic piano experience with portability and headphone privacy. Other electronic keyboards can have hundreds or even thousands of functions, including the ability to program straight into a home computer. As both digital pianos and "synth" keyboards come with USB interfaces, however, their functions often overlap.
While digital pianos may imitate exactly the sound of a traditional hammer piano, traditional orchestras and jazz ensembles continue to prefer the piano over the keyboard. Popular and much jazz music, however, relies on the keyboard not only for synth piano, but also for other musical effects and on-the-spot mixing. Some musicians use keyboards to become one-person rock or even symphonic bands.
Prospective keyboard students have a number of options. Some private teachers will let you bring your own keyboard into their home or studio; others prefer that you take lessons on their keyboard, and practice on your own. Some teachers and classes take the view that the keyboard is essentially a piano, and should be learned similarly; others focus on keyboards' multi-modal or technical aspects. You can also learn how to play and use keyboards yourself, with packages that combine sheet music, audio CDs, and streaming video.