You see event signs like this all the time, and we print lots of them. The material is called coroplast, although we hear people call it corrugated plastic or even plastic cardboard.
Typically they are 18×24″ in size, and held up with a wire H stake.
These event signs are not for long term outdoor use, but they are fine for most events. We see customers using golf tournament signs year after year, since they are only displayed for a day or two each year.
We consider the life of a coroplast sign to be a maximum of six months, which works just fine for a lot of applications, particularly event signs.