Ubuntu 12.04 was released on the 26th of April 2012, which just so happened to be a Thursday. From there on in, Ubuntu 12.04 Precise Pangolin provided millions of users all over the world with a free, stable, secure operating system.
Ubuntu 13.10 was released on the 17th of October 2013. This newer version of Ubuntu replaces 13.04 as the latest release, and is available alongside the LTS release, Ubuntu 12.04 Precise Pangolin.
An .iso file is an image of a CD, DVD, or BluRay disc. It is a way of storing files in a computer as if they were on a disc. Unfortunately, individual files are not accessible easily, but can be extracted like a zip archive.