My Dad always told me that good papers make for good friends...Always get it in writing.
 
What is a contract: A contract is any legally-enforceable promise or set of promises made by one party to another and, as such, reflects the policies represented by freedom of contract. In the civil law, contracts are considered to be part of the general law of obligations. This article describes the law relating to contracts in common law jurisdictions.
 
What is a customer: Groups or individuals who have a business relationship with the organization--those who receive and use or are directly affected by the products and services of the organization. Customers include direct recipients of products and services, internal customers who produce services and products for final recipients, and other organizations and entities that interact with an organization to produce products and services.
 
When you're dealing with customers, sometimes things can go wrong. It might be your fault, it might be their fault or it might be no-one's fault -- but if you didn't make a contract, then you'll all suffer.
 
A contract gives you a sound legal base for your business, and some guarantee that you're going to get paid for your work without you having to ask the customer for payment in advance. In the event of a dispute, the contract lays down what the agreement was so that you can point to it and say what was agreed. If you ever end up having to go to court (let's hope you won't), the contract is what the judge's decision will be based on.
 
You might think that you're never going to get into a dispute with your customers, but it's all too common to find yourself in a little disagreement. They will often want to get you to do some 'small' amount of extra work to finish the job or make it better, not realising that doing so would completely obliterate your profit margin.
 
If you are doing high-value work for some clients, though, it could be worth the time and trouble of having your lawyer write a formal contract, or at least of doing it yourself and getting a lawyer to look it over. Formal contracts will give you more protection if the worst happens, and there's nothing to stop you from making it a one-off expense only by re-using the same contract for multiple customers.
 
Luckily, it isn't usually so necessary to be paranoid about contract law with small purchases anyway, since customers will be paying you first and receiving the goods or services afterwards, not the other way around. If you plan to offer any kind of payment plan or other long-term agreement, of course, this should always be backed up with a signed contract.