With Ubuntu 14.04 Trusty Tahr rolled out you must be wondering why Canonical Ltd, the developer of Ubuntu operating system named it Trusty Tahr. Canonical releases a new version of Ubuntu semi-annually and each version is given a codename which is a combination of an adjective and an animal with the same first letter. The version number denotes the year and the month of the release.
Trusty Tahr - an adjective + an animal with the same first letter. 14.04 signifies that it was released in 2014 in the month of April.
Ubuntu Naming Conventions
LTS is Long Term Support i.e. Canonical will provide updates to that version for a period of 5 years and paid support is available. Each forth release is a Long Term Support release or you can say the 1st year release in the even year is a Long Term Support Ubuntu release.
If we leave the first two versions, the rest are in the alphabetical order. You can read a detail release information of each version of Ubuntu on Wikipedia.
Other Linux distributions like LinuxMint also has codename logic. Read here: Version numbers and Codename logic in Linux mint.