While developing a Ruby application or while learning ruby, one of the things you must use is IRB (interactive ruby). As in its man page is said "irb is a tool to execute interactively ruby expressions read from stdin.". In this tool you can type and execute directly ruby code. It's very useful but like most other programs like ViM (Vi IMproved) the real power is its customization.
Here I post my .irbrc and to make things clear there are some explanations on each line.