Ecards is the shorthand term for electronic greeting cards, which is what helps make it a popular tag.
Electronic greeting cards are like normal greeting cards or postcards except they are delivered and viewed electronically. Hundreds of free and pay web sites offer a wide variety of ecards. Generally, the sender can customize their selection to a certain extent. They provide the e-mail address of the recipient, who will get an e-mail from the site with a link to the ecard itself.