Email is a method of exchanging digital messages. Unlike Postal mail ("snail mail") it is fast and free to use.
You can often get an email account from your internet service provider. And you can always get a free email address from places like Google (Gmail), Yahoo, or MSN.
Ecards are greetings that are sent to email addresses. There are still a few places that will allow you to send ecards free. Most of the good ecards are now from subscription services, but they are are still a lot cheaper than buying and mailing paper cards.