When you use Apple’s Mail app on your iPhone to send email, the default signature is “Sent from my iPhone.” The iPad is not much different as it’s default signature is “Sent from my iPad.” If you’d prefer to change it to something more personal and not advertise the fact you have an iPhone or iPad with every email you send, don’t bother poking around in the Mail app itself. Instead, go to Settings > Mail > Signature, where you can change the signature to anything you like. You can also delete it entirely. If you have multiple email accounts configured, such as one for work and one for home, you can also set a different signature for each of these accounts in the Mail settings.
Have an iPad? This also works well on the iPad!