Full dentures remain in place by form fitting to the gum ridges and creating a suction bond. Partial dentures often have additional ridges that fit around the natural remaining teeth helping them to stay in place.
It can take several visits to your dentist to be properly fit with a set of dentures. The dentist will take several impressions of your gums, and send these to a dental lab which will make the actual appliances. Once they are complete, a second appointment with your dentist will help to ensure that they fit well and that your bite is comfortable.
One consideration that you and your prosthodontist should discuss is the danger of resorbtion of the underlying bone. The bones in which our teeth are anchored are maintained by the stress created by chewing. When the natural teeth are removed the underlying bone may go into a kind of atrophy and actually begin to be absorbed by the surrounding tissue. This process is similar to what can happen to muscle tissue when a patient wears a cast on an arm or leg for an extended period of time.
The titanium posts that form the base of dental implants create the same kind of stress that natural teeth do, negating the possibility of resorbtion.