help with decode

Can anyone tell me how the ||'s are interpreted in the DECODE below? I was thinking string concatenation, but shouldn't there be commas after 'M' and 'T'?
Many thanks for your help.

SELECT
        DECODE(NVL(template_id,0),0,'M' || mailing_id,'T' || template_id),

        from A Received on Wed Aug 03 2005 - 08:53:56 CDT