According to About.com, every wedding invitation requires the same basic information, including who is getting married, where the wedding is occurring, the date and time of the wedding and when RSVPs are required. If there is a reception after the wedding, that information should also be included with the invitation.
Traditionally, wedding invitations also include other information, usually written in a formal manner. Often wedding invitations name the hosts of the wedding and reception; the hosts are traditionally the parents of the bride. The invitation also includes the actual statement of invitation, in which the guest is asked to join the hosts at the ceremony; traditionally, the hosts ask for the "honor" of the guests' company if the service is to be held at a church or synagogue, and the "pleasure" of their company if the ceremony will be held elsewhere. The names of the couple getting married are also included.
On formal invitations, the date and time of the wedding ceremony are spelled out in words, rather than using the more standard numerals. The location must also be included, with an address given if necessary.
Information regarding the wedding reception is usually included with a wedding invitation, sometimes on a separate card. It is polite to let the guests know what type of food will be served at the reception. While RSVPs are always requested, sometimes hosts also include RSVP cards with stamped envelopes to make it easy for guests to respond. It is never acceptable to request or even mention gifts on a wedding invitation.