How to "tag" records?

Hi – I was wondering whether you had a facility to make “tags” for certain conditions. I would like to be able to email groups of clients with specific conditions and am finding this difficult to do at present.

What I would like to do is to add some tags to each client e.g. autism or gluten free or allergy and then be able to search for everyone on the database with these conditions.  I then need to export this list so that I can send an email or a mail shot.

Please let me know if this is possible and how to do it.

Many thanks
 
Lucinda