The Fermi observatory supports a variety of different types of observations. The most common mode for the spacecraft is all-sky survey, where the observatory scans across on hemisphere of the sky for one orbit, then rocks to the other hemisphere to scan for the second orbit. This mode of operations provides about 30 minutes of livetime at any point on the sky approximately every 3 hours (two orbits).
The observatory has three different modes of pointed observations: autonomus repoints occur in response to interesting gamma-ray bursts, a target of opportunity pointed observation may be requested by any member of the Fermi science community, and pre-scheduled pointed observations may be part of the normal science timeline.
In addition, the observing plan may combine several different observing modes. For example, the observatory can switch between pointing at different targets, or, more commonly, will perform a portion of sky-survey while the pointed target is occulted. More complete descriptions of the various observing modes are available at the links below.