Of course the meaning of some signs is less obvious than the meaning of others. On the International Roadsigns subpage I've placed images of the more important and confusing signs. Diamond signs indicate priority. Red triangles are warnings. Red circles are restrictions.
Blue circles are requirements.
Squares and rectangles give guidance. Note the signs which show two arrows pointing in opposite directions. If one of these arrows is red, it means the traffic traveling in that direction must yield to traffic traveling in the other direction. The color red on a European road sign signals negative information such as a warning or prohibition. For another instance check out the sign that means No bicycles.