What exactly does the Unix bash command do when run without options

Question

When you type bash into the terminal and press enter, you go into what looks like an interactive bash interpreter... As far as I know, Terminal pretty much is, anyway.

The only visible difference is that the command prompt line says

bash-3.2$

instead of

Marcos-MacBook-Pro-3:Desktop marcoprins$

So what is happening when you run bash without options?