Invitation letters are used for both personal as well as business purposes. Though these letters serve the same purpose as an invitation card, the fact is that invitation letters are more personal and generally contain additional information or details.
While starting off an invitation letter, mention what the reader is being invited to. The name or type of event, date, time and venue are the basic facts that any invitation letter must contain.
The letters can also mention the expected dress code for the occasion. You can also include details like what the program would entail and mention if the invite includes lunch/dinner/cocktails etc. All these additional inputs can help the readers to come better prepared for the occasion.
If you so wish, you can extend the invitation to any personal guests or family members of the person you are addressing it to. In case of occasions where you do not want to receive any gifts, be polite in mentioning so.
Before you sign off, remember to include RSVP details, as it will give you a fair idea of who is and who is not attending.