An invitation letter is always more personal than a printed card.
An invitation letter can be used for both business as well as personal use.
It could be a job invitation letter or an invitation letter for a business event.
An example of a personal invitation letter would be an invitation for a Christmas party or an invitation for a wedding.
It is important to give the time, date and venue of the event or party for which you are inviting.
It is also preferable to give a RSVP contact, so that you can be informed of the confirmation or regret in relation to the attendance.