Bathtub liners are new surfaces for your bathtub (this is very similar to a shower wall/pan liner).  In generaly, a bathtub liner is installed on top of your existing bathtub in order to avoid the labor intensive work that goes along with resurfacing your tub.  In general, bathtub liners are built from thick acrylic, which is what most modern bathtubs are built from.  While bathtub liners are an affordable short term solution, they do present many problems, primarily that they make your tub smaller and can allow water & mold to be traped between the liner and the original surface of your tub.  The advantages of bathtub liners are that they are far more affordable than a full remodel (like refinishing, they avoid all of the demolition that goes along with a bathroom remodeling project)