A viewing means to have an open casket.
A visitation is when the body is laid out in the casket (which may be open or closed) before the service so that mourners may come to "visit."
A visitation offers a chance for people to "pay their final respects" to the dead person.
Just as important, the visitation can be a time for mourners 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.
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 arrange 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 prayers for the dead led by a mourner or member of the clergy.
The term wake is sometimes used to describe the reception after the funeral or memorial service.
Traditionally, though, a wake means the friends and family keeping watch by the body before it is safely buried