The most common variety of magnetometer one is likely to find in the field, as opposed to a Proton Magnetometer. They are used to conduct geophysical survey (most often with archaeological objectives).
A fluxgate gradiometer is a formidable piece of equipment. It takes the form of a sophisticated (read: temperamental) passive sensor for use in ground-scanning. It will measure, quantify and record local variations in magnetic flux density, measured in Teslas (rather than magnetic field strength, measured in Webers, which would make it a mere metal detector). Due to the subtle nature of most sub-surface archaeological remains, magnetometry readings are usually output in NanoTeslas (nT) and fluxgate gradiometers are capable of resolutions ranging from 100 to 0.1nT. Readings are relative; that is to say that the data gained is not an absolute value of the ground's magnetic flux density, but rather the degree to which it differs from a pre-determined zero point. Finding a suitable "zero" datum is usually the first thing to be done on arriving on a site to be scanned.
Fluxgate gradiometers do not need to come into contact with the ground to take readings. The most common configuration is a piece of equipment held rather like an upside-down "L". Having properly oriented the equipment with relation to the earth's magnetic field, calibrated it, and zeroed it (which can take anything from 5 to 45 minutes, depending on daily local conditions), the operator will generally walk the length of a grid square at a steady pace, taking timed-trigger readings as he goes (a "traverse"). He will progress along the square at regular intervals until he reaches the end. A typical method might be to scan an area divided into 20x20m squares, taking readings every half-metre, and spacing the traverses at 1m intervals (this would give twice the resolution in the direction of the traverse, and a total of 800 points of data).
Magnetometry has advantages over most other geophysical techniques in that it is passive, extremely quick, and requires only one operator. This contrasts with the other popular technique, resistivity, which is both active (placing electrodes into the ground) and very time consuming. Data can be downloaded from the machine to a laptop computer, and results can very rapidly be plotted on screen. It's quite exciting to find a neolithic "jelly-baby" house and associated midden just by pacing the area a few times.
Another thing in favour of the Fluxgate Gradiometer is that it has a really cool name.