This document discusses invitations and replies to invitations for social events. It provides information on:
1) Invitations can be formal or informal, with informal invitations sent to close friends/relatives in a personal letter format.
2) Replies to invitations are sent to thank the host and indicate whether the invitee will attend or not so the host can plan accordingly.
3) Samples of informal invitations and replies are provided to demonstrate their relaxed tone and personal nature compared to formal versions. Tips are given for writing effective informal invitations and replies.