We use a Bartington 601 Gradiometer. We chose this one because it has a 1m separation between the sensors which increases sensitivity, the downside of which is that you detect metal fences easier too. It is quite easy to set up and use. The main problem we have with it, is being able to read the instructions on screen when we've taken our glasses off  to reduce our magnetic signatures!

The theory side of things with a magnetometer is basically quite straightforward, the detail  and the application is complicated! The principle of the machine is that at either end of the vertical pole is a sensitive magnetic field sensor. The one at the bottom is affected more by magnetic features in the ground than the one at the top, simply because the one at the top is further away from the ground. The instrument records the difference between the two sensors in the box near where the machine is held. The box at the back is a battery to power everything.

In use you just walk along at a steady pace and the machine records how far you've walked and the difference in signal from the sensors at suitable intervals. Sensibly the machine bleeps when it thinks you have traveled 1m and we have tapes on the ground with 1m marks which helps synchronise readings. Maintaining a steady pace can be difficult if the vegetation conspires against you.

Software converts these numbers into colours or shades of grey so you can see shapes appearing in the results. The things that a magnetometer detects are those with a strong magnetic signal, every cast horse shoe or nail made of iron will be a blip, every gold brooch will be missed! Archaeologically, burnt things such as kilns or hearths or even bricks to some extent, have a strong signal. Local natural rock used for building won't show up but for some reason the natural processes of decomposition produce magnetic signals, so ditches that have been open for a while before infilling show up, whereas graves that are dug and filled in soon after don't. What you end up with is a picture like this:

Magnetometer Image

which shows a portion of a circular ditch next to a rectangular ditch enclosure.
The white area at the bottom left corner was due to a nearby metal wire fence.
More technical discussion here.