The easiest way to describe a trioval is to imagine a triangle composed of three straight lines that are connected by curves (or rounded points) rather than sharp points. Other people refer to them somewhat D-shaped.
The best examples of triovals are probably Daytona International Speedway in Daytona, Florida and Pocono International Raceway in Long Pond, Pennsylvania. Typically a trioval will be considered to have 3 corners or turns, although some racing traditionalists will count up to 5 or even 6 distinct corners. Nazareth Speedway in Nazareth, Pennsylvania is one such case, though I only count 3 turns from looking at the track from overhead and count 4 turns from driving it. I guess that it all depends on what line you run.