Quick Guide to Semiotics - Barthes

Semiology deals with anything that stands for something else. It is the study of signs and symbols as elements of communicative behavior.

Sign = Signifier + signified

This equation is perhaps the easiest way to remember what a sign is. The signifier is the physical form a sign takes, and the signified is the concept behind that physical form. For example, the bathroom door sign is made up of the stick image of a man, and that image connotes that this door is for males to enter. Combining both the signifier and the signified, the sign is complete and we understand it to be a washroom for males.

A sign is not alone, it is part of a system. The study of semiotic systems is called taxonomy.