vCard is a file format standard for electronic business cards. vCards can contain name and address information, phone numbers, e-mail addresses, URLs, logos, photographs, etc. vCards are often attached to messages because:
See vCard on Wikipedia for more details about this format.
To automatically attach your vCard to all outgoing messages:
Rather than automatically attaching your vCard, you can attach it to individual messages. While composing the message, click the arrow near the Attach button and check or uncheck the Personal Card (vCard) menu item.
When a vCard is attached to a message, the message will have an attachment with a *.vcf extension. Thunderbird will automatically recognize and preview this attachment in the message body, as shown below:
Click the icon to open the vCard and add it to your address book.