Definition of Logarithms

I have to give a talk on microbiology to a group of adults (meat
industry staff) with no previous experience in this area. I love doing
these courses but I'm guilty as sin of dropping into jargon from time
to time and making assumptions about peoples level of education.
One thing previous participants have had problems with is the notion
of log10 figures. I have been using standard math definitions to
explain it, but many of these people simply do not have the background
in math to understand what I am saying.

Does anyone have a good plain english explanation of what log10 means?


Many thanks

Des Walsh