Difference between Contact and Attendee
Contact is any person who registered to at least one of your events and was added to the system. Obviously, contact can register to and attend several events as any real person can do.
When a contact registers to an event, they get an attendance record (or attendee for short). Attendance record contains a detailed information about the participation of the contact in a single event and their performance. For example, billing information, payments, evaluation, certificate, etc.
When the same contact registers to another event, they get another attendance record (or attendee for short). So in the system there will be just one contact, but two attendees.
Contact
Contact profile contains information about all your events a person registered to.
If it's the first time a person registers to your event, a contact profile is created along with an attendance record. If this person has registered to any of your events before using the same email address, they already have a contact profile and the new event they registered to will be added to this contact profile.
Contacts are available in the list in the top menu.
Each contact is associated with a particular email address. If a person registered to several events of one trainer using one email address, all these events will be added to one contact profile. If the same person uses 2 different email addresses to register to events, 2 separate contact profiles associated with each of the addresses will be created.
From a contact profile, you can switch to an attendance record of a particular event.
Attendance record (attendee)
Attendance record contains all information related to a contact's participation in a particular event. A new attendance record is created for each event a contact registers to.
The list of attendees is available for all types of accounts.
Trainers and certification brands can access the list of all their attendees from the top menu or from the Attendees tile for a particular event.
A training company can open the list of attendees for a particular event from the Attendees tile.
All types of accounts can edit personal data and participation information or delete a record completely).
Merging of contacts
Attendance records exist separately from each other and can't be combined in any way.
Contact profiles can be merged. For example, if you have several contact profiles with different email addresses for one person, you can merge them. After merging, all participation history and all email addresses will be under one profile.
Read more about merging of contacts here.