Orchestration in music can have two meanings:
It can mean: the study of how to write music for an orchestra.
It can also mean the list of instruments that are used in a particular piece of music.
People who want to compose music for orchestras need to study orchestration. They can do this by reading books about orchestration, but also they can do this by studying orchestral scores and listening to orchestras in concerts or on CDs. Students at music colleges will also practise orchestration by taking piano music and arranging it for orchestra.
The word instrumentation means almost the same thing as “orchestration”. “Instrumentation” means: understanding each instrument and knowing about all the sounds they can make. It can be about any music with instruments, whereas “orchestration” refers to the orchestra from its beginnings in the 17th century to the present.
Orchestration is all about understanding how to write well for different instruments, how to combine them so that they sound well together and balance well.
Sometimes composers leave the orchestration of their works to other people. Leonard Bernstein is famous as the composer of West Side Story, but although he composed all the notes he left it to other people to orchestrate. The composer Maurice Ravel took a piano piece by Modest Mussorgsky (who had died forty years earlier) called Pictures at an Exhibition and arranged it for orchestra. Most people know this version rather than Mussorgsky’s piano version.