the hsc online site has some OK infomation on gravity data reduction...
The jist of it is this; there are two main types of corrections Temporal (time) and spatial.
Temporal- Earth tide correction accounts for the changes due to the tides
-Instrument drift- accounts for the changes due to the instrument moving while the reading is being taken
Latitudinal- Accounts for the fact that the Earth isnt totally spherical
Bougier height - accounts for the surrounding terrain always subtracted because if the reading on the top of a mountain there is less nearby pulling the meter dwon and if the reading is taken from the bottom of a valley then the mountains on either side are pulling the meter up.
Free air- Accounts for the height of the meter
Topographical- Accounts for the wider terrain surrounding the place that the reading is takem from
I can't gurantee that this is totally correct since it was all off the top of my head but it should be mostly right.