Symbols are things like national flags, or the colours in a set of traffic lights. Icons are a bit like symbols, except that they look like the things they represent.
This green man means you may cross the road. But the red man means you shouldnt go. We can see the signs in a set of the traffic lights. This one means Turn right. It shows that there is no straight road, you can only turn right.
This icon means dollar. In a lot of countries dollar is used so its known. This sign means that something is dangerous for your life. This sign means that here mobile phones are not allowed.
This or this sign means love. This sign means man. This sign means woman.
This symbol means that smoking in this place isnt allowed. Symbols and icons are so common because people from different countries understand them.