Viral marketing means to spread out information. Traditionally this was and is, still done via   word of mouth.  Online it can be achieved with emails, newsletters, promotions and your website (to name a but a few).

For example, I received an email the other day from a friend who had forwarded it to me.  The message was about the Take That Circus Tour DVD being launched on 24th November.  There was the option to opt in in the email to receive information.   My friend forwarded it to me, because we went to the concert together and she knew I would be very interested in the email.

My friend was obviously on a companies email list to have received the message. By offering the facility to forward the message to friends and share, the company are building their list.  Because there is a very strong chance that I will sign up to receive more information.  Hence the viral nature.

Viral marketing is free and takes less then 5 mins to set up.  What are you waiting for, add a forwarding facility onto your online marketing and grow your list.