An email signature is an important way to include your contact information with a professional touch to your emails.

The Art And Science Of The Email Signature is a useful article on designing your email signature.

Gmail

How to Add an Email Signature in Gmail 

iPhone

There aren’t as many features for creating an email signature in iPhone settings.  One way to copy your email footer to your iPhone settings is to send yourself an email from your regular email program. Then you can cut and paste the signature from the message into the iPhone settings screen.
Create an iPhone Email Signature

Adding a Logo to an iPhone Signature

Other Email Programs

Each email program has a different way of creating an email signature, but the process is fairly similar.  To find instructions on adding an email signature in another program, just do an Internet search on the program name and “how to create an email signature.”