Change Control and Quality Assurance (QA)
Suppose the customer changes his mind about the color of the paint halfway through the process of painting the house. The change must be recorded, because the customer will probably have to pay extra for changing his mind, and the schedule must be changed because it will take longer to get the house ready. The change also must be communicated to everyone concerned, so that all affected areas of the house are repainted and everyone's schedule is adjusted appropriately.
Similarly, if the IT customer changes his mind about the look and feel of the system, the change must be recorded, the project might have to be rescheduled, and it would be important to ensure that the customer's changes were reflected throughout the system.
When building a house, it's important that all the work be checked (quality assured or QA'd) at many points during the building process, and then thoroughly inspected and okayed by an expert before the customer is told that the house is ready to occupy. In the IT world, it's equally important that the different components of the system be QA'd frequently during the development of the system, and then subject to a thorough check before delivery to the client.