A location context specifies a circular geographical area in which it is active. It is formed by a centre point (latitude, longitude) plus a radius around that point.
The most accurate provider available (around +-10m), but its performance degrades quickly indoors and it uses a relatively large amount of power.
To set the frequency of GPS fixes, see Menu / Preferences / Monitor. GPS Check Time determines the check frequency in seconds while the device is awake, Off Check Time while the device is asleep.
GPS will only be used for a particular context if it is specified in the location edit screen.
This provider uses a combination of cell-towers and WiFi information (if WiFi is enabled) to determine the device's geographical location. It is less power-hungry than GPS, but also less accurate (sometimes +- several kilometres) and requires data network availability.
Network will only be used for a particular context if it is specified in the location edit screen.