The Pedal Harp
The harp pictured above is a version of harp you most likely saw played in an orchestra. The pedal harp has seven pedals near the bottom of the harp (you can see two of them in this picture). Each pedal corresponds to a note on the scale - C, D, E, F, G, A, or B. The pedals each have three positions, sharp, natural and flat. This is how the pedal harp affords a harpist access to the chromatic scales and key changes found in modern music.
Before Johann Sebastian Bach came on the music scene in the early 1700's, much of the music around was diatonic, or using a 7 note scale. Composers tended to pick a key and stick to it within a piece of music. Bach wrote music that used more colors of the scale (more than 7), changed keys in the middle of pieces and opened up a whole new way of writing and thinking about music.
Consequently, harps at that time had a hard time keeping up with this music. Harps are well made of diatonic music because they have seven strings to a scale. But they had to retune between musical pieces to change keys. Instrument makers created many different types of harp (cross strung, double strung, triple strung and pedal) to keep up with the chromatic needs of the evolving orchestra. By the mid to late 1700s, instrument makers solidified the pedal harp design and by the turn of the century the double action pedal harp became the typical harp used in the orchestra.
I adore my pedal harp as I have access to music that I cannot play on my other harps. Personally I love it best for playing romantic style music from the 1800s and modern music from the 20th century.