Charles Sanderes Pierce developed and extended the theory of Semiotics. He broke it down into 3 different steps, Icon, Symbol and Index. It is important to note that a sign can be more than one of these at once.
These are signifiers that have some sort of physical resemblance to the things that they signify. This category would include photographs, as they have a great deal of resemblance to the things in the photographs. Illustrations would also be counted (mostly) as Iconic Signs.
Illustrations would also be counted as Iconic Signs. An example would be a stick man which which would be an iconic sign as they resemble the humans they signify. Rivers and roads on a map can also be counted as icon as they keep the same shape but are scaled down versions. Spoken words that are onomatopoeias, such as “bang” and “whoosh” are iconic because the sound resembles the sound it signifies.
Here is an example of icons going down to its simplest forms. The hologram of a person is very iconic, so is a photograph, painting, drawing, while a stick person is only just iconic of a human.
These are signs where the signifiers are learnt. They are cultural symbols that have no natural resemblance. This is what Saussure meant about signs being arbitrary. Most spoken language falls into the category of symbols because the words are cultural conventions, not instinctive noises.
Written words are always symbolic because letterforms, and indeed numbers and glyphs too are cultural conventions. Way someones dresses, logos (icons within them).
Indexical signs are ones where the signifier can only exist because of the physical presence of the signified. These might include footprints, smells, animal or musical sounds, fingerprints. Another classic example of an index is smoke, which can’t be produced without fire.
Lipsticks traces on a glass, footprints, fingerprints are all examples of indexes. Also weather fits into the index category; rain, lightning, heatwaves, fog and rainbows which all indicate weather conditions.
This interview helps explains semiotics with clear examples of the different categories Charles Sanderes Pierce developed.