Product or service details: Specify Description, Quantity, Rate, Amount and Subtotal. The more details you include on the invoice, the better. This way, your client will know exactly what they are paying for upon receiving the invoice. Make sure that your client can understand every item, so that as soon as they receive the invoice, they can pay you instead of asking questions and delaying the process.
Along with the payment terms, you can specify whether you charge a late fee for invoices paid past their due dates. Some freelancers like this option, to enforce getting paid on time. You may also want to include your tax ID number in this area for tax purposes.