The harp is a fairly ancient instrument, and instruments have been depicted on the ancient buildings of Egyptian Pharaos.The difference between harps and the guitar and violin families is that a harp has a string for each pitch, unlike the other families where a few strings are used to generate the entire pitch ranges of these instruments. Pitches on these instruments are achieved by shortening the length of the string by pressing on it, typically wsith the fingers. On the violin and guitar, once the finger stops pressing down on the string, the vibration stops. On a harp the vibration does not stop, and needs to be stopped vibrating through the playing technique of the harp. Harp-like instruments are found in many different cultures, and called by names such as the psalterion, magadis and sambuca, the African mangbetu, the Burmese saung, and the Chinese chang.
Harps have sound boxes to increase the loudness of the soundwaves.
The double-action pedal for the harp was invented by Cousineau in the 1700s. The pedal allows for key changes, extending the pitch range possibilities of the harp.
Source: http://www.miayf.org/string/plucked-string-instruments/harps/
Source: http://en.wikipedia.org/wiki/File:Saung_Mandalay.jpg
http://www.youtube.com/watch?v=cU1VFErmUGA