Difference between single quotes and double quotes

Question

I have two variables declared in shell script

var1=23
var2=27

echo $var1              # Prints 23
echo '$var1'            # Prints $var1
echo "$var1"            # Prints 23

From the above I have some confusion regarding single and double quotes usage on variables.

please can any one clarify this with an example difference between single and double quote.