shell - What is the difference between "$a" and $a in unix

For
example:

#!/bin/sh
a=0
while
            [ "$a" -lt 10 ]
 b="$a"
 while [ "$b" -ge 0 ] do
 echo -n
            "$b "
 b=`expr $b - 1`
 done

 echo

            a=`expr $a +
            1`
done*

The
above mentioned script gives the answer in triangle while with out the double quotes, it
falls one after the other on diff lines.