Many companies start off with a basic brochure type website and then once they have started getting in enough business, look to upgrading their site to an ecommerce one.
An ecommerce site allows the visitor to purchase a product or service online and pay for it through a payment gateway. There are many payment gateways but the popular ones are Sage, Worldpay and PayPal.
If you are considering doing this then speak to a website programmer who will be able to let you know the pros and cons for most of them and which will be the best option for your site. Some of the payment gateways are more complex to integrate which means will cost more upfront but allow more flexibility which may be useful in the near future. If you chose a simpler method, you may find that a few months down the line you need to start again with a new payment gateway integration.