I don't really know much about freelance business, but in the actual business practice, there are basically a few ways. The safe way is as below:
1. Client ask you to do something
2. You quote the client the price, delivery date, payment method and other terms & condition inside a 'Quotation'
3. Then when the client agree to the quotation, then either they pay u a deposit or you just directly go n produce the work/product for them.
4. When you give your final actual work to them, give them the invoice at the same time. Inside the invoice is usually the description of the work done and also the payment / remaining payment required.
5. Wait to collect your cheque.
It's just my 2cents from my experience but if you really want to play safe, a contract will be the best.
thanks.. i think i have a clearer understanding now..
currently doing the same way.. xD