Often Google Images can be helpful with questions like these. "Sign" is generally used for street signs (that have the name of the street on them or other directions for drivers) or restaurant/store signs (that have the name of them on the signs). In general signs are larger. Placards are generally smaller plates posted onto something else (like a door or wall) with information on them. Notice generally refers to actual pieces of paper with an announcement or warning. It is common to get a billing notice to remind you to pay your bills. Flyers are pieces of paper or brochures or pamphlets or cards that people hand out to tell people about events or organizations.

A Sign = gives directions or information. A Placard= very similar a sign placed in a public place or a sign held by protesters at a demonstration. A Notice = gives information like a sign. A Flyer = a small sign that is used to advertise an event often they are handed to people on the street or distributed through peoples doors or posted on walls or in public places. so there is some overlap and interchangeability, but flyer is specifically used to publicise an event, although sometimes it may/might publicise public information etc. They may have different meanings in other English speaking countries, this is for the UK and other people may slightly disagree with me.