whats does movsx instruction do?!

Hi guys...!
I have searched on the net about this question but i could'nt find a good help...!
However, in the follow source i can not understand what does movsx do?!!!
Who can help me?! and describe for me?!!! Please

.section .data
        milad:
        .int 20
.section .text
.globl _start
_start:
        nop

        movl $0, %eax
        movl $0, %ebx
        movl $0, %ecx
        movb $20, %al
        addb $10, %al
        movw %100, %cx
        addw %cx, %bx
        movsx %bx, %ebx
        movl $100, %edx
        addl %edx, %edx
        addl milad, %edx
        addl %edx, milad
        movl $1, %eax
        movl $0, %ebx
        int &0x80