A
hot tub originally was a round wooden tub shaped like a barrel.
These became more and more popular in the 1960's. They were
very simple, basically only holding hot water and only enough
room for one or two people at a time.
As
this part of the leisure industry began to increase, hot tub
shells were made out of fibreglass or thermoplastics. The name
then changed to Spa, this differentiated them form the wooden
type
A lot of spa's/hot tubs we see today have many different layers
(for thermal properties) before usually being finished by an
acrylic shell, although the traditional hot tub is all wood.