What's the difference between Coroplast and foam board signs?

The core difference is the material and where each one survives. Coroplast is a waterproof plastic for outdoor use and rough handling, while foam board is a paper-faced foam meant for clean indoor display.

Rule of thumb: if it might meet weather or repeated handling, choose Coroplast. If it stays indoors and needs a smooth rigid surface, foam board gives you the cleaner face, for example trade show graphics or gallery signage.