What is the use of space in shell?

view story

http://stackoverflow.com – I have a shell program as follow: #!/bin/sh a=1 b=2 c=`expr $a+$b` echo $c The output is 1+2. But if I change c=`expr $a+$b\` to c=`expr $a + $b` The output is going to be 3. I wonder what is the use of space here. (HowTos)