There are four key objects in Resova that every API developer should know about.

Objects
items An activity/serivce is known as an Item, these are what your customers will purchase.
instances An instance is a date and time belonging to an item. An instance is required to be included when creating a booking.
bookings A customer will make a booking for a specific Item and Instance (date/time), they're essentially purchasing something. All bookings belong to a Transaction
transactions A Transaction is the overall purchase order. All Bookings belong to a Transaction and there can be more than one Booking per Transaction.