A viewing means to have an open casket. A visitation is when the body is in the casket (which may be open or closed) before the service so that friends and family may come to "visit." A visitation offers a chance for people to "pay their final respects" to the deceased.
Just as important, the visitation can be a time for family and friends to meet and console each other in a more informal setting than at the funeral. You can schedule a visitation for as little as half an hour on the day of the service, or it can last for several days before the service. It is important to consider the wishes of the deceased, if known, and the wishes of the family. The visitation can be restricted to just close friends and family, or be open to the public. You can even have a combination of private and public hours.
As part of the visitation, you can have a formal ceremony, an informal ceremony, or none at all. The formal ceremony might be a brief service with the saying of the rosary (for Catholics) or a Legion service, or some other Fraternal Organization.