[dba-SQLServer] Multi-field vs field indexes

Can someone write (or point to) a description of single field vs multi-field
indexes.  Let's take an example of city/state/zip.  If I created a single
index for all three of these fields, would the index be useful for the
individual fields as well.  If I were to do a join on Zip, would the
multi-field index be used to select the zips for the join or would a single
field zip index be needed in such a case?
 
John W. Colby
Colby Consulting
www.ColbyConsulting.com