Eclipse Simultaneous Releases Callisto vs Europa

[Vyoma 0]

Lately, I have started using Eclipse for both my job and the other opensource development projects I am invovled in. It turns out to be good not only for Java development, but also for PHP development (after installling PDT plugins).Getting it installed in my Ubuntu boot, I noticed that there were only Callisto version in the package manager.Anyway, this piqued my curiosity. What are these simaltaneous releases? What is the difference between Callito and Europa releases of Eclipse? Are there any documentations that I can refer to?