What are the spiral arms of the milky way made of?

In short, those arms are made of "stars, dust and gas". They are visible as spiral arms because they have a higher density of those objects than the space between them.
 
The arms are an area where the density of stars (and planets and other objects) is unusually high and where the start are young - recently formed- and this also gives those starts their visibility as young starts are also very bright.