Semiotics is the study of sign systems. It explores how words and signs make meaning. In semiotics, a sign is anything that stands in for something other than itself.
The word semiotics dates back to ancient Greece, but its use in modern language was introduced in the 19th century with the research of Ferdinand de Saussure. Saussure was a Swiss linguist who contributed a lot to the study of semiotics, also sometimes referred to as semiology.
An Icon has a physical resemblance to the signified, the thing being represented. A photograph is a good example as it certainly resembles whatever it depicts.
An Index shows evidence of what’s being represented. A good example is using an image of smoke to indicate fire.
A Symbol has no resemblance between the signifier and the signified. The connection between them must be culturally learned. Numbers and alphabets are good examples. There’s nothing inherent in the number 9 to indicate what it represents. It must be culturally learned.