Poster boards look similar in a photograph and behave differently under a marker pen. The substrate is worth knowing before you buy.
Paper card
Thin, light and cheap. Fine for a short-lived display that will be pinned flat. It will bow if stood upright unsupported and it will cockle if you use wet glue or heavy marker.
Corrugated board
A fluted layer sandwiched between two liners. Much stiffer for the weight, which is why most trifolds use it. You can see the flute direction on the cut edge — the board is far more rigid across the flutes than along them, so orientation matters if the board is wide.
Foam board
A foam core between paper faces. The stiffest of the three and the most pleasant to mount work on because the surface is smooth and does not telegraph texture through a photograph. It dents if something sharp meets it and the dent is permanent.
Choosing
Match the substrate to the medium. Anything wet — glue sticks are fine, PVA less so — is happier on foam or corrugated than on card. If the board is being reused across several events, spend on stiffness; a floppy board looks tired after one outing.