eCard is the shorthand term for electronic greeting card, which is what helps make it a popular tag.
An electronic greeting card is like a normal greeting card or postcard except it is delivered and viewed electronically. Hundreds of free and pay web sites allow users dozens of choices in selecting an appropriate eCard. 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.