I do not know exactly where to post this because I don't know exactly what I
am asking for.  I have noticed that a number of websites, that allow readers
to comment on articles, protect themselves from automated responders by
generating a graphic image of random numbers and letters that would be hard
for anything other than a human to read.  People are required to enter that
random combination in a submission box before their comments are posted.

I don't know what this technique is called, but I want to find out how it is
done so that I can add similar protection to a website that I am working on.

I am hoping that someone here will know what I am talking about and give me
a place to look.

Thanks.