terminal app on my iphone 5?
Local
To use a local terminal, you have to jailbreak.
At which point you would install mobileterminal(with cydia) to access the local shell.
Remote
For accessing other systems, you would need a ssh client (although you could do the above, and run ssh).
I recommend prompt.
Also, if you jailbroke your iPhone and installed openssh, you could just use prompt to connect locally; it has a better interface than mobileterminal.