Signifier: any material thing that signifies, e.g., words on a page, a facial expression, an image.
Signified: the concept that a signifier refers to.
Together, the signifier and signified make up the:
Sign: the smallest unit of meaning. Anything that can be used to communicate.
Symbolic (arbitrary) signs: signs where the relation between signifier and signified is purely conventional and culturally specific, e.g., most words.