Symbols, indexes and icons are categories within this range. Each of these categories vary in their degree of abstraction. They don't look like the objects or concepts that they signify.
Icons are at the other end of the spectrum.
Indexes fall between symbols and icons in their level of abstractness. Distinctions are not often easy to make. A map for example, can be argued to be a symbol, an icon or an index with equal justification. Effective communication does not require us to differentiate between these types. An understanding of what sets them apart can, however, make our attempts at communication more effective.