A Collection is a set of Transactions. The relationship is, Collection has many Transactions. You can choose to create a Collection either from API or within your SecretPay's dashboard. An example of Collection would be something like Tuition Fee for June 2015, Donation for Earthquake, Ticket Payment, and many more.
A Transaction represents the promise made to you by your customer. It's an invoice for your customer. A Transaction must belong to a Collection. To start using the API, you would have to create a Collection. Then the payment flow will kick in as per below:
Normal Completion Flow