You might have different types of client, including members of the public ('business to consumer' or B2C) and businesses ('business to business' or B2B). These could be local, regional, national or even international - and they could well be a mix.
Depending on your location and the nature of the business you might build up a personal relationship with many of your clients, or you might rarely see the same client twice. However many times you deal with an individual client, the best form of advertising is word of mouth recommendation, so it is very important that your business provides a first class service to every client, no matter how small.
How will clients pay
For many office-based businesses it is still normal to receive payment by cheque. It's fairly unusual for most office-based business to receive regular payments in cash, although some will receive cash from time to time. Where this is an unusual occurrence it is important that systems are in place to ensure that the payment is correctly recorded and that the cash can be stored safely.
You may decide to accept debit and credit card payments, including payment through Paypal or similar services - this is becoming increasingly commonplace in professional firms. If you provide services for large clients on a regular basis you might also accept electronic payments.