RPM packages have a way of indicating what exactly they provide as part of their installation. These "provides" are usually calculated automatically, but can also be added manually by the packager. Usually, this is done in order to satisfy generic dependencies -- for example, some packages may require a way to send mail, but they don't really care whether you run postfix, sendmail, exim, or whatever. To solve this problem, packages that provide mail transport capabilities have a generic "Provides: MTA".
You can check on your system:
yum provides MTA
You will see that this returns exim, postfix, and sendmail. When multiple packages provide a generic functionality such as "vnc server", they can include that as part of their "manual provides." In our case:
yum provides vnc-server
This will return several hits, of which tigervnc-server is one.
You can list all "provides" in a package by using
rpm -q --provides. E.g.:
# rpm -q --provides tigervnc-server
config(tigervnc-server) = 1.1.0-6.1.fc17
tightvnc-server = 1.5.0-0.15.20090204svn3586
vnc-libs = 4.1.3-2
vnc-server = 4.1.3-2
tigervnc-server = 1.1.0-6.1.fc17
tigervnc-server(x86-64) = 1.1.0-6.1.fc17