Bash aliases are essential for anyone using the command line. An alias is basically just a shortcut for some other, typically longer command.
Sendmail is an open source smtp server. Many different languages and frameworks interface with sendmail, and when debugging it's good to be able to quickly test whether or not sendmail is working.
It's nice to have quick access to your command history on the command line, but sometimes it's valuable to know exactly when you typed a command.
As a developer, I've customized my bash prompt to always show the current git branch if I'm in an initialized git repository. This is convenient, but it can make for a very long bash prompt.