Posters express a message that people can understand in less than 2 minutes without interpretation. In Pinellas County 4-H a poster measures 14"x22", contains a 2-dimensional large graphic, and little writing (a sentence at most). Posters like these are usually done for the poster art and dairy poster contests.

Visual aids can be any size. They can contain charts, graphs, pictures, large font text, etc. They are not completely understandable without someone telling you what it is about. Also, visual aids may be 3-dimensional models or actual items. A visual aid is used during a demonstration.

An exhibit is used to show others something about a project.  An exhibit may be an actual object or done on a poster board, is 14"x22".  The poster board would contain some small graphics, and a lot of text. You can expect to spend at least 5 minutes reading the text. You can also add 3-dimensional objects like samples of hay. 

Additional Resources 

http://florida4h.org/events/files/2008-2010_Events_And_Activities_Handbook_low.pdf