Above-ground fiberglass models are framed with metal or plastic, similar to vinyl-lined swimming pools. Concrete pools are sometimes lined with fiberglass instead of plaster. Why? Fiberglass requires less maintenance than plaster.
A fiberglass pool or spa can be assembled on-site, say, in your backyard, from panels. Some of these panels are an entire molded shell, while othes are fiberglass walls mounted on a concrete pool bottom.
Most spas are made of a type of fiberglass shell, which can be a spa shell placed directly into the ground to a small portable unit.
Source: The Ultimate Guide to Pool Maintenance (Terry Tamminen; McGraw-Hill Co., 2007, third edition).