How It Works
The harp is a unique instrument and many elements of how it is constructed bring light to how to play or write for the instrument. The modern harp has 47 strings and 7 pedals which raise or lower each pitch-class of strings. What that means is that the harp doesn't have any 'black notes' like the piano but rather pedals which sharpen or flatten each string of the same pitch.
When the pedals are raised as in the pictures below and to the right the strings are in their flat position and the pitches of the strings are: Cb, Db, Eb, Fb, Gb, Ab, Bb, etc. When you depress each pedal to the middle position at the top of the harp disc 1 turns, and the pins stretch the string slightly, raising the pitch to a natural. Each pedal controls a different pitch so by altering the C pedal only from the example above you would have C, Db, Eb, Fb, Gb, Ab, Bb etc. Depressing the pedal further will turn disc 2, streching the string even further and thus raising the pitch so we have C#, Db, Eb, Fb, Gb, Ab, Bb etc. Each pedal controls all the string of that pitch (so by depressing the C pedal you change all the Cs on the harp.)
This means that harpists are constantly changing their pedal positions whilst playing the instrument and in the rests. Often they'll have lots of changes to make, particularly if the piece changes key dramtically. For example if they were moving from D major (with the F and C pedals sharpened) to B major the harpist would have to sharpen 3 more sets of strings: G D and A. Pedal positions are marked in harp notation and you'll often see harpists in an orchestra checking their pedals with their feet whilst watching the conductor for their entry: if the pedals are wrong then the audience would definitely notice!
The pedals are designed with key changes in mind so that each additional sharp or flat comes on the opposite side of the harp.
Harp strings are made of metal gut and sometimes nylon, and are coloured to give the player a visual aid to find the right string, with all the Cs red and all the Fs Black.
The lowest strings are made from steel and the rest of the strings are made from gut, although some players use nylon strings for the very highest notes, as gut strings tend to break more often.
About an hour before a concert you'll see the harpists on stage tuning their harps - with 47 strings it can take a long time!