What is the difference between sign and signal?

Signals can move—blink, rise through the air; signs not so much.

Signals alerts to present or future conditions; signs can be seen in hindsight.

A Sign is a clearly visible object, generally flat, bearing a short message in words or pictures. 

  1.   A traffic sign.
  2.   A meaningful gesture.
  3.   An astrological sign.
  4.   A specific gesture or motion used to communicate by those with speaking or hearing difficulties; now specifically, a linguistic unit in sign language equivalent to word in spoken languages.
  5.   Sign language in general.

A signal is an indication given to another person (a radio, TV, telephone, internet, etc.).

To signal – to indicate.

For example:

My mobile phone can’t get a signal in the railway station.

The police officer signaled me to stop.

Signals by authorised persons 

 ​You must obey signals given by police officers, traffic officers and traffic wardens.