I am listening to a piece of music sung in maqam bayat (or bayati). It's Psalm 126, which is 7 verses long, but the piece takes about 10 minutes and sounds very improvisatory. This is similar to other examples of music in a maqam, so I'm wondering: is this type of music, where a soloist sings slowly, repeating words, without a repeated melody, mostly improvised, or is it mostly composed ahead of time and performed similarly at different performances, whether by the same singer or a different singer? I unfortunately know very little about how Arabic classical music works. It's possible that the example I'm citing, which is actually Jewish music from the Arabic world, is exceptional. Can anyone tell me more about Arabic classical music and improvisation?
A great deal of it is improvised, much in the same sense that Indian Raga is improvised. That is to say, a lot of melodic framework and development is predetermined, but there is a lot of room to work around the predefined bits.
http://en.wikipedia.org/wiki/Arabic_maqam explains this well.
Also: This is question is too wide. You should remove the bits like "Can anyone tell me more about Arabic classical music and improvisation?" - that is not well suited to the format of the site.