An act is essentially a group of contacts. For instance, your band would be an act and have its own contacts. Your solo acoustic show would be an act as well and have its own contacts.
A contact is a person who is part of an organization. For instance, a bar owner would be a contact and the bar would be organization to which he belongs. A booking agent or music journalist could also be a contact.
A collaborator is someone who works with you when you are booking an act. For instance, if 3 people in your band all book shows for you, then you can add them all to your act and you can work together.
A booking is an event that happens at a specified date and time that is agreed upon with a contact. Bookings can be shows, interviews, opening acts, festival gigs - just about anything!