Globular clusters are spherical in shape and contain many tens or hundreds of thousands of stars. They are found all over the sky.
Open clusters tend to have a more irregular shape. They contain dozens, hundreds or perhaps a few thousand stars at most. They tend to be found embedded in the Milky Way.
A quirk of how hydrogen atoms behave allowed astronomers to discover the spiral structure of the Milky Way. Radio emissions from the atoms in clouds of gas enable us to calculate their velocity and along with some reasonable assumptions about the Milky Way's rotation, their distance from us can be inferred. When astronomers made these measurements and calculations the spiral structure of the Milky Way emerged.
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In terms of the Milky Way the density wave is made from stars, gas and dust. In a density wave there is more material available to star formation. Stars are created in the density wave and in optical wavelengths the spiral pattern is traced out by the presence of massive stars. But the stars orbit faster than the wave and soon leave it behind. The lives of massive stars are relatively short (a few million or tens of millions of years) and they explode....whilst behind them the density wave is helping to create further new stars.
The spiral density wave theory is not yet set in stone but is the best model we have at present to explain the long-lived nature of the spiral arms.
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The origin of the spiral structure itself is not completely understood. There are suggestions that gravitational interactions with one or more dwarf galaxies could have perturbed the disk to get the spiral structure started.
The Milky Way is undrgoing one such interaction at the moment - with the Sagittarius Dwarf Galaxy (discovered in 1994).
Exploring the Cosmos: An introduction to the history of astronomy (lecture 17) by Dr. Barbara J. Becker
The Milky Way Galaxy (Lecture notes from Durham University)
Interstellar Medium and the Milky Way (Nick Strobel's astronomy notes).