why these regsiters

From: Bill Cunningham (nospam_at_nspam.net)
Date: 05/18/04 

Date: Tue, 18 May 2004 16:09:45 -0400

    I would like someone to explain to me why certain regsiters are used for
such and such a purpose. For example.

   .data
    .type len,1 /* variable len of one byte */
.globl main
    main:
        mov eax,1
        mov ebx,len

I don't know what I've just wrote except for the data section for variables.

    Bill