Question: To make an alias for the Terminal in OS X, you can either put the aliases in .bash_profile or .bashrc. What is the difference between the two and why would I choose to put aliases in one and not the other?
Answer: .bash_profile is executed for login shells, while .bashrc is executed for interactive non-login shells.
When you login (type username and password) via console, either sitting at the machine, or remotely via ssh: .bash_profile is executed to configure your shell before the initial command prompt.
But, if you’ve already logged into your machine and open a new terminal window (xterm) then .bashrc is executed before the window command prompt. .bashrc is also run when you start a new bash instance by typing /bin/bash in a terminal.
On OS X, Terminal by default runs a login shell every time, so this is a little different to most other systems, but you can configure that in the preferences.