While conflicts were implemented in earlier versions of RPM they never worked properly until RPM 2.3.4 (well, we hope they work properly now anyway).
Conflicts allow a package to say it won't work with another package (or virtual package) installed on the system. For example, qmail doesn't work (without custom setup) on machines with sendmail installed. The qmail spec file may codify this with a line like:
Conflicts: sendmail
The syntax of the ``Conflicts'' tag is identical to the syntax of the ``Requires''
tag and conflict checking may be overridden by using the --nodeps
flag.