The Invitation List is the list of all the people who you send an invitation to with the event response form link. This is your ’mailing’ list which contains information on whether the person viewed your invitation and whether they responded. Even if you send multiple invitations, your consolidated invitation list enables you to easily further correspond with the people you invited.
The Guest List contains all the people who have responded to your event. Some of these people may not be on your original 'invitation list' because a person on your invitation list included them as their guest, or the invitation was forward to them, or they responded by clicking a 'direct link' to the event response form.