When you create any type of transaction within CRM, you will create an invoice. And invoice simply means "financial transaction." It's our terminology for transactions.
Invoices also give you the ability to enter multiple donations in one transaction for a contact for those situations where they pay for both with the same payment method. You can also use invoices to record multiple transaction types - ie, a Membership and a Donation - and record that they were paid for together.
If you have a donation you don't yet have payment for, it should be entered as a Pledge instead.