It is a truism that a representation is anything that represents something. Thus words, sentences, thoughts, and pictures may all be considered representations, though the manner in which they represent things is very different.
Representation is a philosophically puzzling relation.
To take one simple example, ' x represents y ' seems to express a relation between two things. Pictorial representation seems initially to be the most straightforward form of representation, since the relation between a picture and what it represents seems so natural and obvious to us. The apparent simplicity of pictorial representation might suggest that it is the most basic form of representation. But explaining pictorial representation in terms of resemblance raises many problems. These sorts of considerations have led philosophers like Nelson Goodman to deny that resemblance has anything to do with representation at all.