There are 2 types of forcefield, linear and radial, these can provide effect like gravity, wind and water.
linearforcefields can simulate a number of very different effects depending on their settings.

Ambient Fields (Gravity)

If a linearforcefield has no size attribute (or it is left blank) then it is an ambient field, and it affects the entire level. Almost all of the original levels have an ambient field providing the gravity.

Local Fields (Wind)

linearforcefields can also be localized by giving them a size and a center. In this case only objects inside the field are affected by it. This is used most commonly to simulate wind or to support the Goo Balls when they are sitting on a cloud.

Water

When the water attribute is set to true, the game displays the water effect within the field in the given color. The top edge of the field is given surface ripples and makes little splashes when objects enter or leave at speed. However beyond these visual differences the field's physics operates in the same way as any other Local field.

force
This attribute sets both the direction and strength of the force field. The standard "gravity" field in most levels is set to 0,-10 => 10ms2 straight down.
dampeningfactor
This affects the speed of the Goo Balls as they move, and can be used to give the effect of moving through water, or some other "sticky" substance.
This is normally set to 0 for most fields, and water fields have values between 0.1 and 0.5.
antigrav
Normally set to true, this attribute allows Goo balls with the antigrav attribute (balloons etc) to float upwards when attached. If this is set to false, antigrav balls do not float and act like normal Goo balls.
type
This can be set to either force or gravity. When set to gravity the field will accelerate all the Goo Balls at the same rate, regardless of their mass. When set to force the field applies the same force to each object, so lighter ones will accelerate faster, heavier ones slower. See video below for a demonstration

radialforcefield

These fields are also used to provide a gravity effect, but on a larger scale. Used in Graphics Processing Unit and Main World View to give the orbiting effect. These fields cannot be ambient, so they must always have a center and a radius specified. The type,antigrav and dampeningfactor attributes work the same as linearforcefields see above

forceatcenter and forceatedge
Since the direction of the force is always towards (or away from) the center of the field, the force attributes only give the strength of the field. Negative values specify a force pulling towards the center point (like gravity), positive values specify a force pushing away from the center.
If forceatcenter and forceatedge values are different, then the force varies linearly between them.

Note on geomonly

This attribute can be applied to linear fields. When set to true the field only affects dynamic geometry objects and not Goo Balls. This can be useful in the construction of mechanisms - [link to...]
Note: geomonly tag was present on all radialforcefields in the original levels, but doesn't have any effect when set to true, so was removed from WooGLE in v0.74