A poster is usually for advertisement or to impose an emotion or to express something, it is also artistic. A place card is informational usually dealing with safety or warning. Such as "Beware of Dog" or "No Parking/Towing Zone"

A poster is a printed picture, sometimes with wording. It can be an advertisement. A placard is a notice or information, put on a large piece of paper or board and shown in public. It may or may not have a picture. Also, a placard can be a name on a metal rectangle, put on somebodies office wall or table to identify them.

A poster and placard are pretty much the same thing, not much of a difference from what I know of. It's just that the word poster is more common.