Email is a means of communicating one on one with another person. When you send an email, it is just like sending a letter. Email always gets there a lot faster. And there's no stamp to put on the envelope.
Email is also a new way of writing.
It tends to be a lot less formal than old-fashioned letter writing. People say what they want to say, and tend to dispense with a lot of the formalities. Which is not to say that there are not formalities at all.