Removable Dentures are an artificial method to replace missing teeth.  Unlike 'fixed' bridgework or Implants, dentures are removable appliances that rest on the gums (gingiva).  The support of nearby teeth is minimal resulting in reduced biting capacity.       
Full Dentures, where the appliance can only rely on the gums can reduce the biting capacity by 90% (compare to natural teeth). That is why many denture wearers can not bite into apples, carrots, steaks etc.   
Partial Dentures use the remaining teeth in the area to help the dentures stay in the mouth. Due to this support from the surrounding teeth, partial dentures are less likely to move or fall out.  The pressure generated on those supporting teeth will eventually cause them to become loose and prone to infections or fractures.
Dentures are ideally used as temporary solution for missing teeth, until a more permanent solution becomes available (i.e. fixed bridge, implants, waiting for bone-graft or gingival healing).   
Dentures are exclusively used in areas where there in no chance for a permanent solution due to severe bone loss or compromised teeth that will neither support Implants or a fixed bridge.