So what's the difference? In the majority of cases there are no differences other than name.
In the early days of solar panels they tended to be small and often were just directly connected to a 12V battery for charging purposes.
Even though the open circuit voltage of these panels was much higher than 12V, directly connecting to the battery made for an inexpensive (albeit inefficient) charger. And since the battery was 12V it was easy to think of the panel as also being 12V.
The true maximum power point
of these panels (and most modern 12V panels) is close to 18V and thus should be considered 18V panels not 12V.
Also, most panels advertised as 24V are really two 18V panels in series with an open-circuit voltage well above 40V.
Both 12V and 18V panels are listed for sale on Amazon and inspection of the electrical specs shows that they are essentially identical.