Relines involve the removal of approximately 2-3mm of the inside acrylic that touches the gum and is replaced with new acrylic refitted to the changes in gum shrinkage that cause loose dentures. 
New technologies have been developed to allow for relines to be done in a day, this eliminates the need to leave the denture overnight. Not all relines can be done this way so an evaluation should be done to decide if this method would be suitable for you.

Rebasing is a similar process where all the pink acrylic is replaced with new acrylic, while still using the old teeth. This can only be done if the teeth are in good condition.