http://www.youtube.com/watch?v=xNXw7hAhP7I
An Interactive visualization on how our own Milky Way, a spiral galaxy, was made...

The Milky Way Galaxy is a good example of a swirling galaxy. It consists of a large mass of stars drawn by gravitational forces into the form of a relatively flat disc. The disc is rotating, with the dense central body of stars moving at greater speeds than those towards the rim of the disc. As a result, the pattern of stars within the Galaxy as viewed from directly above or below the disc has formed into a spiral. Because of localized gravitational variations, the spiral pattern has itself formed several distinct spiral arms that radiate outwards, where particularly large numbers of stars can be found. The Sagittarius Arm (also known as Sagittarius-Carina Arm) is one of four major spiral arms extending from and around the central hub region of the Milky Way Galaxy.

The Sagittarius Arm is divided into two parts. Curving outward from the galaxy's central bar is the Sagittarius bar, which further outward becomes the Carina arm. Sagittarius Arm's innermost end connects to one of the ends of this central bar, making it one of the two major spiral arms of the galaxy. Each spiral arm is a long, diffuse curving streamer of stars that radiates out from the galactic center. These gigantic structures are often composed of billions of stars.

The dense, inner arm of Sagittarius is located between the Scutum-Crux Arm and the Orion Arm. The Orion arm (also known as Carina-Cygnus arm) is marked as the Local Spur on the galactic map; it also contains our own Sun. It is named for its proximity to the Sagittarius constellation as seen in the night sky from Earth, in the direction of the galactic center.

More than a million galaxies have been counted so far. Galaxies in the universe had enormous range of properties, from the young and old, all the way to the big, small, luminous and faint. Galaxies don't always form swirl shapes, our large spiral Milky Way galaxy is just one type of galaxy. The spiral arms are an optical illusion caused by dense hydrogen clouds that block out the radiation from stellar nurseries (were stars are born) in the spaces between the fingers of the galaxy. The black, dead areas are precisely where new stars are forming.

But, to better understand the dynamics of the swirling galaxy we can rely on the definition by "lessons.astrology.com" on spiral arms:

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These arms will cohere until the gas and dust has condensed into individual stars and these stars are dispersed throughout the galactic plane. In fact the whole sequence, if we could speed up the time process, would appear as a spinning pinwheel shedding sparks or stars as it whirls. The new-formed stars stay embedded within the spiral arm where they were born until galactic rotation forces them to migrate and be scattered through space, dissolving the spiral arms. New arms are continually being formed.
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http://g.astrology.com/course/astrology_space/AD-11.jpg
The whole structure of the Milky Way galaxy rotates in a clockwise manner.
Source(s):
http://www.astronomycafe.net/anthol/w8s2.html

http://wiki.answers.com/Q/Why_don't_star_clusters_form_the_characteristic_swirl_shape_like_galaxies_usually_do

http://lessons.astrology.com/course/show/The-Astrology-of-Space/3051-Spiral...

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