Ecards is the shorthand term for electronic greeting cards. Because some of the hundreds of sites offering a wide variety of ecards charge and others do not, finding free ecards makes the phrase a popular tag.
Electronic greeting cards are like normal greeting cards or postcards except they are delivered and viewed electronically. Even with free ecards, the sender generally 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.