What you see in the pictures are expanding waves of star material after the explosion. The explosion occured about 11,000 years away in our own Milky Way galaxy. The expanding cloud is now approximately 10 light-years across, and continues expanding at a speed of 4000-6000 km/s, with jets shooting out at 5,500-14,000 km/s in different directions.
The expanding shell has a temperature of around 50 million degrees Faranheit. The different colors seen in color-coded images are representations of matter of different chemical compositions.
The star is calculated to have exploded around the year 1667, although nobody recorded an explosion at the time.The original star is believed to have been 15-20 times larger than our Sun.
An image slideshow as well as a 3-D flythrough, in the video.
Sources:
http://en.wikipedia.org/wiki/Cassiopeia_A
http://spider.seds.org/spider/Vars/casA.html
http://hyperphysics.phy-astr.gsu.edu/hbase/starlog/cassa.html