An invitation is a formal or informal written or verbal request to attend an event. Formal invitations typically include details like the host's information, event details, RSVP information, and closing. Invitations can be in the form of cards or letters and vary in formality depending on the event. Card invitations are usually brief while letter invitations are written like a formal letter. Replies also follow set formats, thanking the host and accepting or declining the invitation. Invitations serve to formally request someone's presence at an event and provide necessary information for them to plan accordingly.