It depends.
With a POP account the sent emails are stored on the iPad. With an IMAP account they may be stored on the iPad, or on the server. It depends on the account and how you have it set up. When you dig through your account settings you should eventually find some Advanced settings, and folder settings. If you can chose between storing sent emails on the server or locally, it will be here. Trash can also be set here, sometimes.
If you have an internet connection, the iPad will send the email immediately. Immediately can take a while. When the email is finally sent the iPad should give you a kind of jet taking off sound, provided you have those sounds turned on.
If you create the email when you have no connection the iPad should send it the next time you have a connection and start the Mail app. It's possible it might send it without starting the app, but I've never tested this and would not count on it.
