It is a (2.5 mile) 9 minute round trip around the periphery of Tomorrowland, across the resort to Downtown Disney and back to Tomorrowland.
It starts when DL opens, and runs until 30 minutes after park closing.
You can only ride the monorail if you have a valid ticket to Disneyland. While the Disneyland monorail serves as transportation from Downtown Disney to Tomorrowland it is more of a ride then a mode of transportation.
The monorail makes two stops. One in Tomorrowland and one in Downtown Disney. They usually make people exit once you get to Tomorrowland, so if you want to go round trip board there.