The earliest form of the harp was a C shaped harp, which gave way to the Lyre invented by the Sumerians around 3200 BC. The lyre had six strings crossing a soundboard and a bridge as well.
The Crwth (crooth) or crit (Irish pronunciation) followed. The Crit had a rectangular frame; the lower half was a sound box, the upper half being left open on each side of the strings. The Crit is considered to be an ancestor of both the harp and the violin.
The Ancient Egyptian harp evolved, and most notable were its size and lack of a pillar. It existed from the Old Kingdom into the Greek-Roman era. It was the Old Kingdom's only stringed instrument (ca. 2575-2134 BC).
Medieval Harps, more as we know the harp today began to be common closer to 400 AD. They were small and portable. Traveling musicians had to carry them on foot or horseback. Materials to make harps were expensive. The shape of the harp and string material of the harp largely depended on what part of the world they were from. Welsh harps were often strung with hair, Irish harps with wire, and Scottish harps from gut. The Irish harps were the first to have a pillar in 400 AD. The famous Brian Boru harp which is wire strung is still intact today at Trinity College in Ireland. At one time Harper's were second in status only to royalty. the term "Folk harp" refers to any non-pedal. Neo-Celtic harps came about with the addition of levers to change the key of of music but kept the shape of the Irish harp.
Multi-Course harps which can be double, triple and cross-strung. The double-strung harp has an ancestor in the ancient "harpa doppia", and today may include levers as well.
Paraguayan, and Latin American harps have a straight pillar and tuning pins much like that of a guitar. They are played by two people; one is a percussionist.
Modern Pedal harps are distinguished from the others in that they change the key of the music by the use of pedals. They are much larger and cover more octaves. They are often used in orchestras having come about in the 1800's in response to symphonic music.