Manual means that the service will start only when Windows or another service needs it, or if you invoke something to start the service. You can use the command
net start servicename from the command line to enable a manual service. When you reboot, the service will again be off until you restart it.
Disabled, on the other hand, means that the service will stay off, regardless of whether you try to start it or not. Other services or applications which depend on the disabled service may fail.
See the MSFN discussion thread on Manual vs. Disabled for more info.