Enter the text of your email in the body of the message. People typically get so many emails in a day that a long email often doesn't get read until later. Sometimes much, much later. You can copy text from another document or email and paste it into a new email, so you don't have to retype text. Many email programs will also let you paste pictures and documents into your message.
Email is more informal than paper mail because it is used with more frequency and immediacy.
Unless it is a personal letter, email usually gets right to the point without a lengthy introduction. Signatures can also include a standard signoff.