Symbol
Is something that stands for or represents something else. Its meaning is derived from a shared understanding. (e.g. a ring symbolises love and marriage)

Sign
An image used to convey information or an instruction. For example, a stop sign.

Logo
A symbol or image that represents a company or organisation, designed to promote instant recognition by a mass audience.

Icon                  
A simple picture or image that symbolises or represents something by suggesting its meaning. Today the term is more commonly used to describe a visual representation on the screen of a device which allows you to access a programme/file/command stored in your computer or mobile phone (eg the image of a rubbish bin on a computer for deleting files)