Every event signup has two things: a registrar that creates the order and an attendee/attendees/sponsor that actually attends the event.
Take for example, a parent that signs their two children for an event. This would result in the following:
Registrar = the parent
Attendee 1 = the first child
Attendee 2 = the second child
Order = contains the registrar's/parent's information as well as references to the attendee's/children's information
Ticket = a person needs to have a ticket assigned to them to be an 'attendee'. In this example, the parent registered and paid, but did not have a ticket assigned to them, therefore they are not an attendee.
Group tickets allow your organization to offer differentiations (such as discounts) from individual tickets by setting the group size.
Sponsorships allow you to choose whether or not to offer tickets or ticket groups while giving users a way to support the event.