ModemManager is a bit of software in some Linux distributions that is probably not needed for many computers.
What ModemManager does
ModemManager is software that, in concert with DBUS, detects when a Modem is plugged into a running Linux system, and performs an automatic configuration to prepare it for use by the NetworkManager of recent Linux releases.
ModemManager is automatically launched by NetworkManager at boot time. It takes up memory and CPU resources and can be safely removed without affecting the functionality of any desktop, server or laptop that does not require dial-up modem.
In releases of Fedora Beginning with Fedora 18, ModemManager can be easily removed. Prior to that, not so much.
To remove ModemManager in Fedora 18 and above, you can stop the service and then use the YUM command to remove the package. It may be necessary to use the kill command or top to manually kill the service.
yum -y remove ModemManager
For older versions of Fedora, if you want to prevent automatic startup, you can remove the file: /usr/share/dbus-1/system-services/org.freedesktop.ModemManager.service.