Flyers vs Posters

The word flyer and poster are commonly used in place of each other, but in fact they are actually two different things.

You should follow the design principles here, but it is also useful to know how the two documents differ from each other.

A flyer is something which is handed out to people, or posted through a door. The most common size for a flyer is A5, although you can get flyers smaller and bigger but no bigger than A4. As a flyer is held by someone and read in their own time, flyers can contain quite a bit of information on them.
Flyers are often discarded by people because they lack an initial impact - so they usually end up on the floor or in a bin!
As much as flyers can contain more, smaller writing - no one really likes reading and they prefer pictures.

A poster is something which is stuck up somewhere. As people might be walking or driving past it (or it might be on a bus going past) then it it needs to have a big impact usually through a short slogan or a image. Whatever is written on the poster needs to be short and memorable so people can glance at it and take it in.

Sometimes posters break their own rules - for a reason!
If the designer knows that a poster is going to be stuck up in a shop window then they know that people can come up to it and read what it says. Therefore some posters might have more writing on that others.
Educational posters normally have more writing on them (some are more or less all writing) but this is because the designer knew it would be stuck up in a school or college somewhere and people would come up to it and read it.